Etymology
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AC

abbreviation of air conditioning, by 1966.

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Definitions of AC
1
ac (n.)
an electric current that reverses direction sinusoidally;
Synonyms: alternating current / alternating electric current
2
Ac (n.)
a radioactive element of the actinide series; found in uranium ores;
Synonyms: actinium / atomic number "
From wordnet.princeton.edu