Modification. Usually done to either a video game console, computer case, video game, or vehicle.
Video Game Console Mod: Qoob Mod Chip for the Nintendo Gamecube which allows a person to run Linux off of the system.
Computer Case Mod: A fiberglass window in the side of the case to see the internal workings of a system, usually accompanied by water cooling, LED-infused cooling fans, and/or cold-cathode lighting.
Video Game Mod: See also: Counter Strike, or any Unreal Tournament 2004 mods.
Vehicular Mod: Cold Air Intake, free-flowing Exhaust work, high-flow catalytic converter, cam shafts, ported and polished upper and lower intake manifolds, specifically tuned Power Control Module (PCM), turbochargers, intercoolers, superchargers, or even engine swaps can all be considered modifications. Also, ricers, those who fall under the category of rice
can consider what they have 'mods'. Shopping-cart style wings, exhaust tips larger than 3", ground effects that scrape speed bumps when you go over them, etc. These types of people are usually frowned upon by the real car modifying community. It is usually prudent to tell them to get a real car, and to learn about what performance really is.