In the music score below, the bottom five parts seem to be string parts. We think the first two are violins, the last is string bass, and the second-last is cello. But what is the third-last? Is it a viola? We understand the clef is an older-style C-clef (alto or viola clef).
Yes, Alto = Viola. It's the French name for the instrument, and the score shown is a part of 'Carnival of the Animals' by the French composer Camille Saint-Saëns.
The other instrument names are easily recognisable. Note that Bb Clarinet is 'Clarinette en SIb'. The French write the scale as do, ré, mi, fa, sol, la, si, do rather than C, D, E, F, G, A, B, C. In German it would be "Klarinette in B'. In Germany, 'B' is 'Bb', 'H' is 'B'. Apart from confusing foreigners, this has the advantage of letting German composers write fugues based on 'BACH'.
Alto clef is the standard clef for viola music. A particularly high passage might move into treble clef. But mostly they live in alto clef.