Voix
French for voice.
What it means
Voix is the French word for voice. It is encountered in scoring indications and work titles, for instance in phrases for voice and piano, and designates there the human voice. Alongside this it stands, as in German, for the individual part of a polyphonic setting. In organ terminology the word appears in stop names, among others for softly sounding string or reed stops that are meant to have a singing character.
Musical use
The double meaning is no accident: the idea of the voice shapes European music far beyond singing. Instrumental settings are divided into parts that are led like vocal parts, and stop names such as those mentioned refer to the fact that an instrument is meant to evoke the impression of human singing. For reading French sources the context is important, because the word alone does not reveal whether a scoring or a part is meant; only the context decides. Voix humaine means human voice; the stop is meant to imitate singing.
Performance practice
With French editions, check exactly what the marking refers to before you plan a scoring. Treat instrumental parts like vocal lines: breathe with them, phrase according to units of meaning, and watch for singable arcs — this is the most useful transfer of the term into practice. At the organ, register stops with a vocal name as a solo colour with restrained accompaniment. With song scorings, clarify which vocal range is meant, since many works exist in several versions.
Bear in mind that in French sources the same word stands for scoring and for a part of a setting: only the context decides whether a singing voice or a line is meant. Check this before you take over scoring indications. For practice it is worth transferring the term to instrumental parts: breathe with them, phrase according to units of meaning, and watch for singable arcs. The voix humaine is a reed stop.
Check, with stop names, whether the designation means a reed voice or a string register, because the sound turns out very differently.
Voix céleste is a string register tuned to float.