The viola ( vee-OH-lə, Italian: [ˈvjɔːla, viˈɔːla]) is a string instrument that is usually bowed. Slightly larger than a violin, it has a lower and deeper sound. Since the 18th century, it has been the middle or alto voice of the violin family, between the violin (which is tuned a perfect fifth higher) and the cello (which is tuned an octave lower). The strings from low to high are typically tuned to C3, G3, D4, and A4.

In musical instrument classification, string instruments or chordophones, are musical instruments that produce sound from vibrating strings when a performer plays or sounds the strings in some manner.