alt (1)

in Internet use (for example alt.right), from the use of alt.* in the newsgroups naming system in Usenet. The term was introduced in 1987 and is said to be short for alternative, as it was meant to be outside the usual newsgroup administrative controls and thus include groups on controversial topics and pornography.

alt (2)

"high tone," 1530s, originally in music, ultimately from Latin altus "high," literally "grown tall," from PIE root *al- (2) "to grow, nourish."

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Definitions of alt from WordNet

alt (n.)
angular distance above the horizon (especially of a celestial object);
Synonyms: elevation / el / altitude