Here's a simple medical definition:

Cephalopelvic disproportion is a disparity between the size of the maternal pelvis and the fetal head that precludes vaginal delivery. This condition can rarely be diagnosed in advance.
Maybe your baby is just not in a good position, I would definitely check out optimal fetal positioning like I said and remember that your pelvis will stretch and open wide and your baby's head will mould. Pelvic scans are not ideal for this reason also, it cannot judge how your pelvis will open and baby's head mould - of course anyone will look at it and tell you that baby wont fit while you are flat on your back and not under the influence of labour hormones. The very least he can offer you is a trial of labour, which is common in most other countries - as the above snipet says, it's rare to be diagnosed in advance. Should you be in the small group who do have true CPD, then you can have an emergency ceasar (just means a caesar after labour has started, silly name for it though). Some labour is better for a baby than none, it gives it some preparation for birth. Having said all this perhaps your baby has moved already, but there are things you can do to help shift baby's position. CPD is mostly misdiagnosed when position is a problem, often posterior babies or another mal-position.