Noun
a clef that puts middle C on the third line of a staff
Source: WordNetThe use of alto clef is usually confined to orchestral first trombone parts, with the second trombone part written in tenor clef and the third (bass) part in bass clef. Source: Internet
Some Russian and Eastern European composers wrote first and second tenor trombone parts on one alto clef staff (the German Robert Schumann was the first to do this). Source: Internet
The trombone occasionally uses the alto clef, but not primarily. Source: Internet