(sub) noun Colloq.
Short for:
1. A substitute.
2. A subordinate or subaltern.
3. A subway.
4. A submarine.

1. Under; beneath; below; as in:

2. Anat. Situated under or beneath, or on the ventral side of; as in:

3. Almost; nearly; slightly; imperfectly: chiefly in scientific terms; as in:

4. Lower in rank or grade; secondary; sub-ordinate; as in:

5. Forming a subdivision; as in:

6. Math. Denoting a ratio, the inverse of a given ratio: The subtriplicate ratio is the inverse of the ratio of the cube.
7. Chem. a Present (in a compound) in less than normal amount: subchloride, suboxide. b Designating a basic salt compound: subacetate, subcarbonate. Also: suc– before c, as in succumb; suf- before ƒ, as in suffer; sug– before g, as in suggest: sum– before m, as in summon; sup– before p, as in support; sur– before r, as in surrogate; sus– before c, p, t, as in susceptible, suspect, sustain. [<L sub- <sub under]