Is it a codec? I don't think so; I believe it uses MPEG4/H.264 for video and AC3 for audio.
Is it a container format? Doesn't seem to make a huge amount of sense; there are enough out there already and it's likely that a suitable one already exists.
Is it simply a specification that specifies a particular combination of pre-existing technologies? But this would imply that AVCHD files could also be called something else, like "MPEG4 container with H264/AC3 inside".
So what exactly does AVCHD mean?