What is the term for a player that does not play regularly but can substitute in for others?

The term "reserve player" is accurate in this context, as it specifically refers to a player who is not a regular part of the starting lineup but is available to step in when needed. This designation reflects a role where the player is often training with the team, prepared to play in games when regular players are unavailable due to injury, fatigue, or other reasons.

A "benchwarmer" also describes someone who spends a lot of time on the bench, but it's often viewed more negatively and implies a lack of participation or skill. A "rookie" is a player in their first year of competition, and while they may also be a reserve player, the term does not encapsulate the idea of substituting for others specifically. A "starter" refers to a key player who regularly plays in the main lineup, which contrasts with the role of a reserve. In summary, "reserve player" precisely describes someone who fills in as needed, showcasing their importance to the team's lineup flexibility.