Term for the distance from the center of a lens to the pt. where parallel rays that pass through it converge
.... http://www.boundless.com//physics/definition/thin-lens-equation 
Boundless... [DOC] [TLE] Focal Point - Convex Lenses - CBakken Home Page [PAR] First, a convex lens is one which is thicker in the center than it is near the edges.