Outside the line, BIONICLE means Biological Chronicle. But what does it mean in-story? For instance, Vakama says "This is the legend of the BIONICLE". Do they call themselves the Biological Chronicle? This has confused me ever since I found out what BIONICLE means.
I don't think the word bionicle is used canonically in the story at anytime. I think vakama just says the story of the bionicle for simplicity and to not confuse kids watching it.
(Here I was thinking this would be a super deep thread about the moral of Bionicle and all that)
But yeah, Majora is right.
Thats too simple, let's think deaper, we knew they partially organic so that could be a reference to that (includes agori because they are organic btw)
But the simple answer is the correct one....
Perhaps it's the word that only the Turaga dares to say out when the time comes, which helps to summary the overall Conclusion at the end of the story.
I always just figured it had something to do with the word "bio-mechanical".
It still means biological chronicle in-story, more or less. The Legends of the Bionicle were the stories the Turaga told relating to Mata Nui's slumber and salvation by the Toa. AKA, the CHRONICLE of Mata Nui's BIOLOGICAL processes fighting back against the Makuta virus. Get it?
The reason this is such a point of confusion is that the whole idea of legends and myths was already long gone by the time they actually revealed Mata Nui in 2008 and could have, potentially, made more clear the meaning of the line's name. As it stands, the only place the I've found that specifically refers to the original meaning of "Bionicle" is here, and of course even if you read this in 2001 the "biological" part would still make 0 sense:
Hmmm, I've wondered this myself too, I simply take it as "bionicle" being a collective name on which every sapient species of the MU fit into, but yeah, as pereki said its probably that.