A chop card is a card that a player is expected to discard if they have nothing else to do. Agreeing to discard from a predictable position is one of the first conventions that new teams come up with, because it allows them to prolong cluing 5s (and other critical cards) until they are actually about to be discarded.
The first known use of the term "chop" was in a 2013 forum post by Ben Smalls in which he recommends that players discard cards from the opposite side of the hand as they add new cards to. He refers to the oldest cards in the hand as being "on the chopping block" and later uses "chop" as shorthand. This strategy, known as Discard Oldest, is ubiquitious and is the foundation for many popular convention systems, such as BGA, Logical Leftism, and H-Group (2016).
Not all convention systems are Discard Oldest. Many Hat Guessing systems eschew the concept of chop altogether, requiring discards to be explicitly communicated. In 2021, Sieve was the first in a wave of Discard Newest systems, including Referential Sieve and Reactor. The Scuola Normale Superiore di Pisa uses a system with Card Cycling, where each player's chop shifts position every time they give a clue.
Discarding chop is a default action, an action they are expected to take if they are unaware of any others. Some systems assign a default action of playing a card instead, such as Permission to Play.
When a player has a useful card on chop, a key question for the team is whether the player is allowed to discard it or can be relied upon to take a different action instead, such as giving a clue. See Permission to Discard for more discussion.