noun

• (music) Of a voice or instrument, high pitch; especially, the octave above the top line of the treble stave.
  "Sop[hy] Moderato! moderato! Madam. Your Ladyship's absolutely in alt. / L[ady] Scr[ape] In alt! Madam? / Sop[hy] Yes, in alt- Give me leave to tell your Ladyship, that you have raised your voice a full octave higher since you came into the room."
• (now archaic) A state of excitement, a heightened emotional condition.
  "I was, however, glad at my heart, that Mrs. Moore came up so seasonably with notice, that dinner was ready. The fair fugitive was all in alt. She had the game in her own hands; and by giving me so good an excuse for withdrawing, I had time to strengthen myself; the Captain had time to come; and the Lady to cool."

noun

• (Internet slang, gaming) An alternate or secondary character.
  "Of these alts, how many of them are a gender other than your own?"
• (Internet slang) An alternate account.
  "You've been here four days and you already know about the incident from last year? You're such an obvious alt."
• (finance) An alternative investment or alternative fund.
  "liquid alts"
• (Internet slang, art) An alternate version of a piece of art, especially without much changes beyond a specific thing.

noun

• (computing) A modifier key used to change (alternate) the function of other pressed keys.

noun

• (biochemistry) Initialism of alanine transaminase.
  "Furthermore, wedelolactone also increased the activities of superoxidase dismutase (SOD) and glutathione peroxidase (GSH-Px) and decreased the level of the lipid peroxidation product malondialdehyde (MDA) in the liver, therefore decreasing the activity of alanine aminotransferase (ALT)."