Locum and Second string

Locum noun - Someone (physician or clergyman) who substitutes temporarily for another member of the same profession.

Second string and locum are semantically related. In some cases you can use "Second string" instead a noun "Locum".

Second string noun - A squad of players that are available either individually or as a team to relieve or replace the players who started the game.

Locum and second string are semantically related. Sometimes you can use "Locum" instead a noun phrase "Second string".