Pocket knife manufacturers make lockback knives in many different styles and sizes. However to be considered an actual lockback pocket knife, the knife will feature a prominent locking mechanism adjacent to the back-end bolster (the lockback “button”), and often times, a single blade. Many times that primary locking blade will be a clip point, but models with a primary skinner blade can be had. The sizes vary as much as the frame styles.
There are very famous models of lockbacks, the Buck 110 (see the “Folding Hunter” description page), the Case Mako, etc. The lockback really made its “claim to fame” when Buck introduced the original folding hunter in 1964, marketing it as a pocket knife that was as strong as a fixed blade.