IIRC (it's been a while since music theory class...), the C between the treble and bass clefs is middle C and is denoted as C4 (the 4th C key from the bottom on an 88-key keyboard). The C an octave below C4 is C3 and the C an octave above C4 is C5.
Likewise the A above C4 is A4, etc etc etc.
This should explain everything for you:
http://en.wikipedia.org/wiki/Scientific_pitch_notation
I play praise bass with a keyboardist from time to time and it's really helpful when he tells me which key and octave he is playing in so that I can know whether to double, or play the lower/higher octaves.
