A pocket clip is the sprung metal strip on a folding knife handle that holds the knife against the edge of your pocket. It keeps the knife at a known position and orientation instead of loose at the bottom of the pocket with your keys.
A clipped knife is always in the same place and the same way up, which means you can draw it without looking and get a correct grip first time. A knife rattling loose in a pocket has to be found, fished out and reoriented before you can use it.
Clips also keep the knife out of contact with keys and coins, which is what puts scratches on a finish and grit into a pivot.
Clip position determines carry orientation: tip-up or tip-down, left or right. A reversible clip has mounting holes for more than one position. A deep-carry clip sinks the knife further out of sight.
Clip tension is the trade nobody mentions: tight enough to hold in a run, loose enough not to shred your pocket seam. Most clips can be gently adjusted, though bending too far permanently weakens the spring.
The term is not academic here. 723 of the products currently on our shelves describe this feature in their own manufacturer specifications, across all nine brands we stock. Examples in stock right now include Boker Trppr Micarta, Gerber Savvy Pivot Lock CF, Böker Plus Kwaiken Button Lock.
Everything below is pulled live from the catalog, so what you see is what is actually in the Woodlawn warehouse today.