1. sub- a prefix, occurring orig. in loanwords from Latin, with the meanings “under,”“below,”“beneath” (subsoil; subway), “just outside of,”“near” (subalpine; subtropical), “less than,”“not quite” (subhuman; suboscine; subteen), “secondary,”“at a lower point in a hierarchy” (subcommittee; subplot).
2. sub- a prefix used in the names of chemical compounds that are bases.
3. sub- a prefix used in the names of compounds in which an element is present in a relatively small proportion: suboxide.
Etymology: (< L, prefixal form of sub (prep.); akin to Gk hypó; see hypo -)
1. (prefix)sub- below or under sub-freezing temperatures
2. sub- less important or smaller a sub-editor on the magazine; a subculture within hip hop music
Definition of 'sub-'
Webster Dictionary
1. sub- 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-)
2. sub- 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
Definitions of sub- 
