Sub and Second string

Sub noun - A large sandwich on a long split roll.

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

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.

Sub and second string are semantically related. Sometimes you can use "Sub" instead the phrase "Second string" as a noun or an adjective.