Pocket turns up in 2 fields: textiles, shop slang. Here is what it means to the people in each one.
Pocket in textiles
A small pouch or bag sewn into or onto a garment for carrying items.
A fabric receptacle built into a garment. Pockets can be patch (sewn flat onto the outside), inseam (hidden within a structural seam), or welt (set into a slit with finished edges). They require careful structural reinforcement at the corners, often using bar tacks, to prevent the weight of carried items from tearing the host fabric.
"Reinforce the top corners of that patch pocket so it doesn't rip when the customer shoves their hands in."
Pocket in shop slang
A recessed area machined into the surface of a solid part.
A blind hole or cavity cut into a workpiece, usually created with a milling machine. Unlike a through-hole, a pocket has a solid bottom and is designed to house a component, reduce the overall weight of the part, or provide clearance for fasteners.
"We need to mill a deeper pocket on this bracket so the bearing sits flush."