DD1 - Transition was the bit where I suddenly realised that it really hurt, and then because I had been told I wasn't in labour, proceeded to freak out and tell my DH that I couldn't do it, and what if they examined me and found me to be half a centimetre dilated and I was going to end up with a C-section (I was actually 10 cm, and ready to push ).
DD2 - Transition was the bit where I was in a semi-conscious daze from the pethidine, but suddenly like a thunder bolt, my head cleared, I sat upright and declared "I've got to push this baby out!" 5 minutes later she was on my chest.
It is a definite change in the state of play, but I reckon it is something that (unless you have a number of similar labours) you are unaware of being so at the time. It would be handy to realise next time...