Voice
A voice is a single melodic line in a polyphonic song, which is usually played by a single specific instrument in an ensemble.
Voices often take the form of different monophonic instruments, meaning that the voice structure is naturally emergent from most cases wherein a structure of several monophonic instruments is used. For instance, a musical culture might have vocals, a plucked instrument for accompaniment, and a sustained instrument (such as a wind instrument) to provide a drone or bassline (along with some assortment of percussion instruments), which would result in their songs generally having three fixed voices.
In Western music
Songs are often specified via a fixed number of voices, such as 4. This historically falls out of choir music, wherein voices were literal voices singing melodies. Notably, voices are considered to be merged if they play the same melody separated by a viable interval of equivalence (such as a fifth, octave, or twelfth), so in order to maintain the relative independence of voices, parallel fifths and octaves are generally avoided in several historical musical styles (although they appear more commonly in modern chord-based music). Alternatively, as Bach did in some compositions, on more flexible instruments (such as keyboards) additional voices may be introduced to make up for the apparent merged voices.
Note that the idea of a fixed number of voices is otherwise retained even in keyboard compositions; 4 voices allows for three to be used for a chord progression and one to be used for a melody
Voice leading
Voice leading refers to the progression of individual voices in chords or polyphonic melodies and their interactions with one another (such as the intervals they create) to create harmony. Voice leading is the basis for a large part of Western harmonic theory. For instance, the resolution from V to I utilizes the leading seventh tone to create a pull towards the tonic, which is a principle also utilized in melodic composition.
