A kill assist is awarded to a player who dealt a fair amount of damage to an enemy player before another team member finishes the assailant off. This feature only appears in Counter-Strike: Global Offensive.
Participating in a kill assist will give the player half of a weapon kill reward in Deathmatch, no matter how much damage the player actually deals.
This feature was added for players who did deal enough damage (but did not kill) an enemy player.
The kill assist is considered to be a minor reward and arguably prevents kill stealing and griefing. In a way, it makes the game more fair for the player who did assist.
To get a kill assistant, "player 1" must deal at least 41 points or more damage, then the enemy is killed by "player 2". If done properly, the kill feed will display "player 2 + player 1 <player 2's weapon kill icon> enemy".
Players can still get a kill assist even after they have died. The score of total kill assists can be seen on the scoreboard.
It is possible for a player to assist in a teamkill in the enemy team, or assisting in an enemy killing your teammate by dealing enough friendly fire damage first. It is also possible to assist in an environmental kill.