1.  os(n = noun.body) - a mouth or mouthlike opening;

2.  os(n = noun.substance) atomic number 76, osmium - a hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known;

3.  os(n = noun.communication) operating system - (computer science) software that controls the execution of computer programs and may provide various services;

4.  os(n = noun.body) oculus sinister - the left eye;

5.  os(n = noun.body) bone - rigid connective tissue that makes up the skeleton of vertebrates;

   A bone. [1913 Webster]

   A mouth; an opening; an entrance. [1913 Webster]

   One of the ridges of sand or gravel found in Sweden, etc., supposed by some to be of marine origin, but probably formed by subglacial waters. The osar are similar to the kames of Scotland and the eschars of Ireland. See Eschar. [1913 Webster]

   The chemical symbol for the element osmium. [PJC]

os, abbr.
1 old style.
2 ordinary seaman.
3 (in the UK) Ordnance Survey.
4 outsize.
5 out of stock.

os, symb. Chem. the element osmium.