Snap

"The entire time I knew Thanos, he only ever had one goal: To wipe out half the universe. If he gets all of the Infinity Stones... he can do it with the snap of his fingers."

- Gamora

The Decimation was a mass genocide initiated by Thanos after retrieving all six Infinity Stones and activating the Infinity Gauntlet: The result was a massive fallout across all civilizations and species in the universe, wiping out fifty percent of all life.

Background
"This universe is finite, its resources finite. If life is left unchecked, life will cease to exist. It needs correction." "You don't know that!" "I'm the only one who knows that. At least, I'm the only one with the will to act on it."

- Thanos and Gamora

In his quest to obtain the Infinity Stones, Thanos invaded Nidavellir and forced the Dwarves to create the Infinity Gauntlet, which was capable of harnessing the Infinity Stones, before he massacred each one of them, except for Eitri. After, he invaded Xandar, destroyed the Nova Corps, and stole the Power Stone from the Orb, which he effortlessly destroyed. Having gained the Power Stone, he slaughtered half of the Xandarians as part of his desire to balance the universe. After having retrieved the Power Stone, Thanos and the Black Order intercepted the Statesman, which was carrying the Asgardian survivors of the cataclysm, Ragnarök. While the Black Order slaughtered half of the ship's population, Thanos personally confronted and defeated Thor, the new king of the Asgardians with Asgard having been destroyed. He then stole the Tesseract from Loki, whom he murdered along with Heimdall. Smashing open the Tesseract revealed the Space Stone as the source of its power. Having gained the next stone, he destroyed the Statesman with the Power Stone, leaving Thor and Asgardian corpses in space. On Earth, Hulk, who was saved by Heimdall's Bifrost Bridge, crashed into the New York Sanctum and warned the Masters of the Mystic Arts of Thanos's threat. Doctor Strange, Wong, Iron Man, and Hulk gathered at the Sanctum and discussed the history of the Infinity Stones' history across the Nine Realms. However, Ebony Maw and Cull Obsidian launched an assault in New York City, prompting the four to fight back. During the ensuing battle, however, Doctor Strange was abducted by Ebony Maw, while Wong sent Cull Obsidian through a portal to the Arctic. Iron Man and Spider-Man managed to sneak onto the Q-Ship. Eventually, Iron Man and Spider-Man rescued Doctor Strange and created a hole in the ship which sucked Maw into space to his death. The three then decided to confront Thanos on his home planet, Titan. In Edinburgh, Vision and Scarlet Witch were ambushed by Proxima Midnight and Corvus Glaive, who attempted to draw the Mind Stone out of Vision's head, until they received combined reinforcements from Captain America, Black Widow, and Falcon. Thanos's two children retreated and the five Avengers traveled to the New Avengers Facility to discuss the Mind Stone. Meanwhile, Thanos personally traveled to Knowhere to obtain the Reality Stone, which was entrusted to the Collector after the conclusion of the Second Dark Elf Conflict. He infiltrated the Collector's museum and located the Reality Stone, which was in the form of the Aether. He then solidified the Aether into the Reality Stone and added it to his Infinity Gauntlet. As the Guardians of the Galaxy arrived on Knowhere to confront Thanos, he conjured a false illusion of him interrogating and torturing the Collector into revealing the location of the Stone, as he accurately predicted that the Guardians of the Galaxy would pursue him to Knowhere. He eventually abducted Gamora and forced her to divulge the location of the Soul Stone with the threat of torturing her sister Nebula with the Power and Space Stones. Thanos's threat forced her to reveal the stone's location, Vormir. Thanos and Gamora then traveled to the mysterious planet, when they were greeted by the Red Skull, who'd become the stone's guardian as a punishment for abusing the Tesseract's power during World War II. After hearing Red Skull's elucidation that he would have to sacrifice Gamora in order to obtain the Infinity Stone, he tearfully threw her off a cliff and was given the stone. On Earth, the Avengers travel to Wakanda as Shuri had knowledge of Vision's stone. However, Wakanda soon was attacked by the Black Order and Thanos's army of Outriders, who demanded the Mind Stone. Acting on Captain America's orders, the Avengers, Bucky Barnes, Black Panther, Dora Milaje, Jabari Tribe, Border Tribe, and Wakandan Royal Guard fought back. Thor arrived on Wakanda with his newly forged weapon, along with Rocket Raccoon, and Groot. The Outriders and the remaining members of the Black Order were eventually defeated and killed. Meanwhile, on Titan, Thanos was confronted by the combined efforts of Doctor Strange, Iron Man, Spider-Man, Star-Lord, Drax the Destroyer, Mantis, and Nebula. Although the seven managed to briefly overwhelm Thanos, he used the power of the four stones to gain the upper hand. A beaten Doctor Strange then relinquished the Time Stone to Thanos so that he wouldn't kill Stark. Having obtained the fifth stone, Thanos teleported himself to Wakanda. In the forests of Wakanda, he appeared out of the Space Stone and used the stones to subdue the Avengers, before making his way to Scarlet Witch, who was using her powers to destroy the Mind Stone, which was contained in Vision's forehead. Although she managed to destroy the last stone, Thanos used the Time Stone to reverse time and restore Vision to his former state so he could proceed to pull the Mind Stone from Vision's head. However, Thanos was suddenly attacked by Thor, who impaled him with Stormbreaker.

Decimation
"Thanos did exactly what he said he was gonna do. He wiped out fifty percent of all living creatures."

- Natasha Romanoff

As Thor denigrated Thanos because of his actions during the Attack on the Statesman and warned him that he would die for murdering Loki, Heimdall, and half of the Asgardians, Thanos simply replied and taunted him that he should have aimed for his head. Suddenly, he snapped his fingers, sending out a blinding light and causing the Mad Titan to pass out. He awakened in the Soulworld where he approached a younger Gamora, who asked him if he completed his mission, to which he solemnly replied yes. Gamora then asked what it cost him, to which he replied "Everything". He then returned to his conscious self and used the Space Stone to transport himself to another location. The consequences of Thanos' actions quickly became apparent, as half of all life in the universe slowly disintegrated into ashes: As Steve Rogers questioned Thor as to Thanos' whereabouts, Bucky Barnes suddenly crumbled into dust in front of Rogers. M'Baku could only watch with horror as Wakandan soldiers around him began to fade. T'Challa, attempting to help Okoye up and encourage her to fight, fell into ashes before her eyes. As Okoye desperately yelled for her King, Rocket Raccoon tearfully watched Groot fade away, weakly calling for his father to help him. Kneeling over Vision's lifeless body, Maximoff peacefully vanished into dust. Hidden in long grass, Falcon disintegrated as James Rhodes called out to him, not realizing his companion was already gone. On Titan, in the aftermath of their defeat by Thanos, the effects of the gauntlet began to claim some more lives, Mantis was alerted to what was happening and soon became the first to vanish followed by Drax and then Star-Lord, Doctor Strange claimed there was no other way to win and soon vanished. Lastly Spider-Man also being alerted by his Spider-Sense succumbed to the effects of the snap desperately begging to live before giving in and dying in Iron Man's arms leaving him and Nebula as the only survivors on Titan. As the effects of Thanos' mass genocide took hold across the Universe, the remaining survivors could do nothing, unable to do anything but accept the reality of Thanos' victory and the consequences it had for them and the rest of the Universe. In Atlanta, Nick Fury and Maria Hill while driving witnessed the power of the Gauntlet claim the lives of the civilians around them, plunging the city into chaos. As Hill disintegrated before Fury's eyes, the former Director hurried to retrieve his last resort Transmitter Pager. Realizing he was disintegrating as well, Fury managed to use to transmit a last-ditch distress call to distant Captain Marvel before crumbling to dust. In San Francisco, Hank Pym, Janet van Dyne, and Hope van Dyne planned to send Ant-Man in the Quantum Realm to extract quantum energy for Ava Starr. Using portable Quantum Tunnel in Luis' Van, Lang made his way to the Quantum Realm and harvested needed energy. However, Pym, Hope, and Janet were all erased from existence, leaving Lang stranded in the Quantum Realm. In the wake of erasing of multiple San Francisco citizens, authority broadcasted a.

Among the trillions of other victims of the Decimation were Sif, Betty Ross, a former S.H.I.E.L.D. agent and an Asgardian actor who portrayed Loki in The Tragedy of Loki of Asgard.

Aftermath
"Some people move on. But not us."

- Captain America

To be added

Trivia

 * The term "decimation" was previously defined as the "killing of one in every ten of a group of people as a punishment for the whole group", deriving from the, and was therefore not necessarily suitable for an event in which 50%, rather than 10%, of a group of people were killed. However, as language has developed, the word has fallen into more colloquial use to often mean merely the "killing or destruction of any large portion of a population", explaining its usage in The Cosmic Quest Volume Two: Aftermath that was used to describe this event.
 * Kevin Feige has said that half of "all life" in the universe was erased from existence, including all sentient and insentient beings.