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: WordNet® 2.0

OS /O-S/ 1. [Operating System] n. An abbreviation heavily used in
   email, occasionally in speech. 2. n. obs. On ITS, an output spy. See
   "{OS and JEDGAR}" in Appendix A.


Source: The Jargon File

OS
     Operating System
     
     
Source: Virtual Entity of Relevant Acronyms