sub

• (informal) substitute; submarine; subordinate; sublieutenant
replace, act as a substitute for, take the place of
under, beneath, below
under, beneath, below; secondary, subordinate; indicating that an ingredient in a compound is present in only a small proportion (chemistry)
a sub is a fixed amount of money that you pay regularly in order to belong to a club or society:an informal word.
a sub is also the same as a submarine:an informal word.
sub- combines with nouns to form other nouns. nouns formed in this way refer to things which are part of a larger thing.
sub- is also used to form words that refer to or describe things that are beneath or lower down than something else. for example, a `subterranean' river flows underground.
sub- is also used to form nouns and adjectives that refer to or describe people or things as inferior, smaller, or less powerful than someone or something else. for example, if a plane flies at a `subsonic' speed, it does not travel as fast as the speed of sound.