Voices: lines that live together
When two or more lines play at once, each line your ear can follow as its own thread is a voice. Nobody has to sing: a bassline is a voice, a lead is a voice, the top edge of a pad is a voice your listener tracks whether you meant them to or not. The word comes from choirs, but the phenomenon comes from how hearing works, and it is the foundation everything called counterpoint is built on.
Two lines, one music
A bass (green, lower) and a lead (blue, higher). Listen to them together first, then solo each one:
Notice two things. Soloed, each line passes the melody test: it has a shape you could follow on its own; the bass is not "accompaniment," it is a slower melody. And together, you can choose which one to follow, the way you can pick one conversation out of a room. That choosing is your ear tracking voices.
What counts as a voice
Anything your ear can follow as a thread. In practice that means a line that is continuous (it moves mostly by steps, or at least by singable moves), stays in its own register lane, and keeps its own rhythm identity. A chord stab is not a voice; the top notes of a series of chord stabs absolutely are, because your ear strings them into a line. This is why voicing decisions in a chord progression are secretly melody decisions: the listener hears the top of your chords as a tune.
Together but separate
The whole art of combining voices lives on one tension: they must sound together (they share a key, a pulse, a harmony) while staying separate (each one followable on its own). Lose the togetherness and it is two songs playing at once; lose the separateness and your two voices collapse into one thick line, which spends one of your instruments for nothing. Exactly how voices protect their separateness while moving, and how they lose it, comes down to four kinds of motion, and that is the next article.
In your music
Count your voices some time: bass, lead, the pad's top edge, a counter line in the mids. That number is a big part of what density measures. When the Theory Aide extension's counterpoint checker opens its track picker, it is asking exactly this article's question: which of your Live tracks are voices, so it can judge how independently they move.