Pad
Pad turns up in 2 fields: PCBs, paper optics. Here is what it means to the people in each one.
Pad in PCBs
Exposed copper area on a board where a component lead or terminal is soldered.
A pad is the copper feature, round, rectangular or custom-shaped, that a component terminal is soldered to, either on the surface or around a plated hole. Its size and shape control solder fillet formation, tombstoning risk and joint reliability. Pads are either copper-defined, with a mask opening larger than the copper, or soldermask-defined, where the mask overlaps the copper edge, a choice that matters most for BGAs.
"Pads on the 0402s are too long, pull them in 0.1 mm or they'll keep standing up in reflow."
Pad in paper optics
Lab talk for a stack of sheets thick enough to be opaque, used for R-infinity, brightness and color readings.
Testers cut several sheets from a reel sample, stack them all facing the same way and read the top sheet with the rest of the stack as its backing. A good pad has the side under test facing up, no folds or fingerprints, and enough layers that one more sheet changes nothing. Readings off a thin pad pick up the instrument backing and wander from test to test.
"Build me a ten-sheet pad, felt side up, and give me brightness and the Lab numbers off it."