Substitute, supply, locum tenens, alternate, understudy, double, stand-in, pinch hitter designate a person who performs or is prepared to perform the duties of another during the latter’s absence or incapacitation.
Substitute is the general term interchangeable with any of the others; specifically it is often applied to a teacher not appointed to a full-time position but held in reserve for service when needed. A supply is a clergyman who substitutes for the regular preacher.
A locum tenens is often a substitute for a professional man with a practice or clientele which needs to be cared for while he is away for a length of time; the term is used especially with respect to physicians and clergymen.
An alternate is one appointed or elected to take the place of another (as a delegate to a convention, a holder of a fellowship, the winner of an award, or a juror) if the latter should be incapacitated or disqualified.
An understudy is a reserve actor or actress prepared to take the part of a regular actor or actress on short notice.
A double is a person sufficiently like another to be able to substitute for him in public and especially an anonymous actor or actress in motion pictures who substitutes in shots or scenes where the required action is considered too risky or onerous for the regular player.
A stand-in in motion pictures is one whose chief duty is to substitute for a star during the preparation (as in arranging lighting) for actual shooting of scenes.
A pinch hitter is a baseball player sent in to bat to replace a weak hitter when a hit is particularly needed or, by extension, a person who substitutes for another in an emergency; the term may connote competence or ability to rise to the demands of the situation but is frequently a more graphic equivalent of substitute .