The answer: not what most people think it is.
I've heard a surprising variety of people misidentify passive voice. Diligent college students. Well-informed writers. Even people authoritative enough to make me question: "Have I studied Latin for eight years and not grasped what the passive voice is? Does it mean something totally different in English?"
NO. No, it does not. They were wrong.
So let's clear this up, once and for all...
WHAT IS PASSIVE VOICE?
By the way, if you already are quite clear on what the passive voice is, and you have friends, family, enemies, or frienemies in your life who get this wrong, feel free to direct them here. So as not to offend them right away, just say it's a post that contains a video of Darth Vader riding a unicycle and playing the bagpipes in a kilt--which they obviously must see.
Talk about universal appeal.
ANYWAY, passive voice.
Definition of passive voice: when the object acted upon (rather than the actor) is the subject of the sentence
Active voice: Darth Vader played the bagpipes.
NOTE: Darth Vader is both the subject of the sentence and the actor of the action.
Passive voice: The bagpipes were played by Darth Vader.
NOTE: Instead of the actor, Darth Vader, being the subject, the object acted upon--here the bagpipes--is the subject of the sentence.
That, and only that, constitutes passive voice. You can talk about semi-passives and passive participles, but that's not actually, necessarily passive voice. Check out UNC Chapel Hill's explanation if you don't believe me. They provide numerous other examples.
And now, I will leave you and Darth Vader to go on your merry way.