Ok, this may be a silly question, but when exactly do the trimesters start and end?

I just got to 12 weeks today and got all excited because I thought we'd now made it into the 2nd trimester, but now I'm not sure if it trimester 2 starts at 12 weeks after LMP, or 12 weeks after conception. I tried looking it up, but all the websites say different things, or else they don't specify what their "12 weeks" refers to...

So confusing.

So if anyone knows for sure, I'd love to know when all of the trimesters start and end