A c-curve is the arch that is created by the natural growth of the nail plate from side wall to side wall. It can be seen by looking down the barrel of the nail from the distal end. Some are naturally quite flat and some can be extreme.
It is a term also used to describe the structure of an enhancement. An arch is a very strong structure and where the c-curve meets the upper arch of the nail or overlay is the apex of the overlay and where most of the strength needs to be created.
It is important to view the natural c-curve when applying tips, and also when applying a nail enhancement, as it forms part of the apex which gives the nail strength.