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American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An advertisement.
2. n. An advantage in tennis.

Century Dictionary and Cyclopedia

1. A prefix of Latin origin, with primary sense “to,” and hence also “toward, upon, for,” etc., expressing in Latin, and so in English, etc., motion or direction to, reduction or change into, addition, adherence, intensification, etc., in English often without perceptible force. According to the following consonant, it is variously assimilated ab-, ac-, af-, etc., or reduced to a-. See etymology.
2. A prefix of various other origin, erroneously put for other prefixes, as in advance, etc. See etymology.
3. A suffix of Greek origin appended to nouns. It is used in forming— collective numerals, as monad, dyad, triad, tetrad, etc., terms used in classifying chemical elements or radicals according to the number of their combining units;
4. A suffix in ballad and salad (formerly balade and salade), usually represented by -ade. See -ade.
5. In anatomy, a suffix denoting relation, situation, or direction, having the same force as the English suffix -ward, or the word toward. Thus, dorsad, backward, toward the dorsum or back; ectad, outward, toward the exterior; entad, inward, toward the interior. So, also, cephalad, headward, forward; dextrad, to the right, on the right hand of, etc. It is used almost at will, with either Greek or Latin words. Its use is advantageous as restricting the idea of direction to the body of the animal itself, without considering the position in which that body may be with relation to externals; since for example, what is backward in the anatomy of man when in the erect posture is upward in that of a quadruped when in the correlatively natural horizontal attitude, while in both it is equally dorsad.
6. An abbreviation of advertisement.
7. A simplified spelling of add.

Wiktionary

1. n. informal Short form of advertisement.
2. n. tennis advantage
3. prep. to, toward

WordNet 3.0

1. n. a public promotion of some product or service
2. adv. in the Christian era; used before dates after the supposed year Christ was born