Alto is a musical term, derived from the Latin word altus, meaning "high", that has several possible interpretations.
When designating instruments, "alto" frequently refers to a member of an instrumental family that has the second highest range, below that of the treble or soprano. Hence, for example, the term "alto saxophone". In other "families", such as the trombone, there is no soprano, the alto having been the highest, although it is absent from the standard modern symphony orchestra.
In choral music, "alto" describes the second highest voice part in a four-part chorus. As well as being the modern Italian word for "high", in the present context it is an Italian abbreviation derived from the Latin phrase contratenor altus, used in medieval polyphony, usually to describe the highest of three parts, the line of which was in counterpoint (in other words, against = contra) with the tenor (which "held" the main melody; this word itself originates in the Latin verb tenere, meaning "to hold").
The alto range in choral music is approximately from G3 to F5. In common usage, alto is used to describe the voice type that typically sings this part, though this is not strictly correct: alto, like the other three standard modern choral voice classifications (soprano, tenor and bass) was originally intended to describe a part within a homophonic or polyphonic texture, rather than an individual voice type; neither are the terms alto and contralto interchangeable or synonymous, though they are often treated as such. Although some women who sing alto in a choir are contraltos, many would be more accurately called mezzo-sopranos (a voice of somewhat higher range and different timbre), and many men countertenors (this latter term is a source of considerable controversy, some authorities preferring the usage of the term "male alto" for those countertenors who use a predominantly falsetto voice production). The contralto voice is a matter of vocal timbre and vocal tessitura as well as range, and a classically-trained solo contralto would usually have a range greater than that of a normal choral alto part in both the upper and lower ranges. However, the vocal tessitura of a classically trained contralto would still make these singers more comfortable singing in the lower part of the voice. A choral non-solo contralto may also have a low range down to D3 (thus perhaps finding it easier to sing the choral tenor part), but some would have difficulty singing above E5. In a choral context mezzo-sopranos and contraltos might sing the alto part, together with countertenors, thus having three vocal timbres (and two means of vocal production) singing the same notes.
Alto is rarely used to describe a solo voice, though there is a plethora of terms in common usage in various languages and in different cultures for solo singers in this range. Examples include contralto, countertenor, haute-contre, and tenor altino among others.
The term alto is also used to designate a specific kind of musical clef. See alto clef.