Source : Webster's Revised Unabridged Dictionary (1913)

Os \Os\, n.; pl. {Ossa}. [L.]
   A bone.

Os \Os\, n.; pl. {Ora}. [L.]
   A mouth; an opening; an entrance.

Os \Os\, n.; pl. {Osar}. [Sw. [*a]s ridge, chain of hills, pl.
   [*a]sar.] (Geol.)
   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}.

Source : WordNet®

os
     n 1: a mouth or mouthlike opening
     2: a hard brittle blue-gray or blue-black metallic element that
        is one of the platinum metals; the heaviest metal known
        [syn: {osmium}, {atomic number 76}]
     3: (computer science) software that controls the execution of
        computer programs and may provide various services [syn: {operating
        system}]
     4: the left eye [syn: {oculus sinister}]
     5: rigid connective tissue that makes up the skeleton of
        vertebrates [syn: {bone}]
     [also: {ossa} (pl), {osar} (pl), {ora} (pl)]

Source : Free On-Line Dictionary of Computing

OS
     
        1. {operating system}.
     
        2. [obsolete, {ITS}], an output spy.  See "{OS and JEDGAR}".
     
        3.  An {operating system} from {IBM} for
        their {System/360} line of hardware announced in 1964.  OS was
        planned with several flavours that were supposed to be
        compatible.  OS was late, memory hungry and not able to reach
        the marketing objectives of IBM for the 360/30, the planned
        successor of the {IBM 1401}.  IBM then decided to design a new
        operating system for the low end machines which they called
        {DOS/360}.
     
        [{Jargon File}]
     
        (1997-09-22)