ac

from The Century Dictionary.

• noun The early form of oak, preserved (through the shortening of the vowel before two consonants) in certain place-names (whence surnames): as, Acton [⟨ AS. Āctū n], literally, oak-town, or dwelling among the oaks: Acley or Ackley, also Oakley [⟨ AS. Ācleá], literally, oak-lea.
• noun A prefix, assimilated form of ad- before c and q, as in accede, acquire, etc.; also an accommodated form of other prefixes, as in accurse, accloy, accumber, etc. See these words.
• etc. Points of flexure in the heating curves of iron and steel. The point ac₁ on heating is the same as ar₁ on cooling, etc.
• noun An adjective-suffix of Greek or Latin origin, as in cardiac, maniac, iliac, etc. It is always preceded by -i- and, like -ic, may take the additional suffix -al.

from the GNU version of the Collaborative International Dictionary of English.

• noun acron. an electric current that reverses direction sinusoidally. Alternative to direct curent, DC.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• conjunction obsolete, dialectal, Scotland But.
• initialism account; money of account
• initialism acre
• initialism air conditioning
• initialism alicyclic
• initialism electricity alternating current
• initialism medicine ante cibum, before meals

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a radioactive element of the actinide series; found in uranium ores
• noun an electric current that reverses direction sinusoidally