Definition of o :
1. A cipher; zero.
2. Among the ancients, was a mark of triple time, from the notion that the ternary, or number 3, is the most perfect of numbers, and properly expressed by a circle, the most perfect figure.
3. An exclamation used in calling or directly addressing a person or personified object; also, as an emotional or impassioned exclamation expressing pain, grief, surprise, desire, fear, etc.
4. O, the fifteenth letter of the English alphabet, derives its form, value, and name from the Greek O, through the Latin. The letter came into the Greek from the Ph/ nician, which possibly derived it ultimately from the Egyptian. Etymologically, the letter o is most closely related to a, e, and u; as in E. bone, AS. ban; E. stone, AS. stan; E. broke, AS. brecan to break; E. bore, AS. beran to bear; E. dove, AS. d/ fe; E. toft, tuft; tone, tune; number, F. nombre.
6. Something shaped like the letter O; a circle or oval.
7. The letter O, or its sound.
f, c, Ark., mm, oh, d, ha ha, h, aitch, e, cipher, Ariz., aorta, meter, alliteration, CA, Ala., boo-hoo, artery, atomic number 8, hmm, ABC's, B.C., iambic pentameter, goose egg, o', group O, atrium, metaphysical, AR, a, AL, AK, b, ABO system, bard, zilch, ahem, beat, conceit, metre, aught, naught, AZ, anticoagulant, type O, metric, achoo, bleeding, oxygen, zip, Alas., ho ho, nothing, hm, metrical, atheroma, g, nil
zero (part of speech: noun)