Pick
Slang
A pick is a type of kill.
The difference between the terms is, that a pick creates a win condition because the resulting amount of players in the enemy team is less than the amount of allied players.
Example:
In a 5v5 situation, should your team get a kill on the enemies so it's 5v4, this kill would be referred to as a pick, instead of just a kill. If the enemies now kill one of your allies so it's a 4v4, it would be denoted as a kill, rather than a pick.