Flank
Flank turns up in 2 fields: fasteners, textiles. Here is what it means to the people in each one.
Flank in fasteners
Either sloping side face of a screw thread, the surface that actually carries load between bolt and nut.
The straight side of a thread profile connecting crest and root. Unified and ISO metric threads have a 60 degree included angle, so each flank sits 30 degrees off the perpendicular, and under load the bolt thread bears on the nut thread along one flank, the pressure flank. Flank angle and straightness errors from worn dies or tools concentrate load on part of the thread and lower stripping strength even when the diameters check good.
"Threads are polished on one flank only, so that bolt's been loaded hard, look for a crack at the first engaged thread."
Flank in textiles
A pocket positioned on the rear seat of a pair of trousers.
A tailoring term for the back pockets on pants or trousers. These pockets are typically constructed as patch pockets on casual wear like denim, or as welted pockets on formal trousers. The placement and angle of the flank drastically affect the visual shape of the garment's seat.
"Make sure the flank pockets are perfectly symmetrical before you topstitch them to the denim."