Dictionary.net

be a substitute; "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet"

A prefix signifying under, below, beneath, and hence often, in an inferior position or degree, in an imperfect or partial state, as in subscribe, substruct, subserve, subject, subordinate, subacid, subastringent, subgranular, suborn. Sub- in Latin compounds often becomes sum- before m, sur before r, and regularly becomes suc-, suf-, sug-, and sup- before c, f, g, and p respectively. Before c, p, and t it sometimes takes form sus- (by the dropping of b from a collateral form, subs-).

A prefix denoting that the ingredient (of a compound) signified by the term to which it is prefixed,is present in only a small proportion, or less than the normal amount; as, subsulphide, suboxide, etc. Prefixed to the name of a salt it is equivalent to basic; as, subacetate or basic acetate.

A prefix to words formed from Latin roots, noting beneath, less than the normal or typical, inferior; it corresponds to the Greek prefix hypo-.

Prefix denoting under.

Under; from under.

Somewhat; slightly; nearly; as, subvertical.

Subordinate; secondary; inferior; lower; as, subcommittee.

A subordinate, of which it is a contraction.

A prefix signifying "under" or "below"; beneath; sub denotes a less or inferior degree, or an imperfect state, of the quality expressed by the word before which it is placed; for the sake of euphony, the b in sub becomes c, f, g, m, p, or s, according to the first letter of the other part of the word, as in succeed, suffer, suggest, summon, supplant, sustain.

In familiar language, a subordinate; one lower in rank.

A common Latin prefix, often signifying' beneath,' or 'a lower degree.'