Kicker
The highest unpaired card in a hand, used to break a tie between two hands of the same category, such as two hands that both hold top pair.
When two hands share the same hand category, the tie breaks on the next card down: first the kicker, then the card below that if the kicker also matches. It is the difference between winning and chopping a pot that otherwise looks identical on the surface, and it is one of the first things a fast board read has to check, not an afterthought once the hand category is settled.