Snap

"Little one, it's a simple calculus. 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

The Decimation was an extermination of half of all life within the universe planned and executed by Thanos who successfully gained all the Infinity Stones and activated them in the Infinity Gauntlet.

Background
"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, just like that."

- Gamora to Thor

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 Space Stone, which was imbedded in the destroyed Tesseract, from Loki, whom he murdered along with Heimdall. 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 Artic. 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 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, Winter Soldier, Black Panther, Jabari Tribe, Border Tribe, and Dora Milaje 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. Although she managed to destroy the last stone, Thanos used the Time Stone to reverse time, and he proceeded to pull the Mind Stone from Vision's head. However, Thanos was suddenly attacked by Thor, who impaled him with Stormbreaker.

Decimation
"What'd you do? What'd you do?!"

- Thor to Thanos

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. He then returned to his conscious self and used the Space Stone to transport himself to another location. Moments later, half of the universe faded from existence, such as Winter Soldier, Falcon, Black Panther, Scarlet Witch, Groot, Star-Lord, Drax the Destroyer, Mantis, Spider-Man, Sif, Betty Ross, Nick Fury, Maria Hill, Wasp, Hank Pym, Janet van Dyne, and billions of others.

Aftermath
"Thanos did exactly what he said he was going to do. He wiped out fifty percent of all living creatures."

- Black Widow to Captain America

To be added

Trivia

 * The term "decimation" is defined as "the killing of one in every ten of a group of people as a punishment for the whole group (originally with reference to a mutinous Roman legion)". This therefore makes the term incorrect as a description for an event in which 50%, not 10%, of life was killed.