The term "Pawn(s)" comes from the game code and refers to any any ambulatory entity that uses pathing. This includes the following:
In colloquial use, this is often used to refer exclusively to a player's colonists. For example, the recommendation that "You only feed cooked meals to your pawns" would likely only be referring to colonists, and would not be inclusive of tamed animals that would also fit under the technical definition of "your pawns".
It likely originates as a reference to both the basic pieces that you move and that move against you, and the simplistic art chosen to represent them.