In an earlier post of mine, this is the way I define peak oil:

On a worldwide basis, the phenomenon of peak oil can be thought of as a crisis in resources needed to produce oil. It's the size of the tap, not the size of the tank. As we deplete the large, easy-to-produce fields and move to ever-more-difficult fields, it takes more and more drilling rigs, more petroleum engineers, and more investment dollars. Eventually we reach a point where we are out of equipment, out of trained personnel, and the investment cost for expanding production becomes prohibitive. When production begins to drop because of all of these pressures, we reach "peak oil".