A flathead
screwdriver is a screwdriver that has a simple, line-shaped "
blade". It fits screws that have heads that look like the
international no symbol (a circle with a line through it). It can be considered the basic screwdriver, as opposed to
Phillips screwdrivers,
Torx screwdrivers,
hex screwdrivers, and other such devices.
Flathead screwdrivers do tend to suffer from slippage, but they also can be improvised -- a butter knife and a flat keychain are two of the things that can be used in a pinch. You can count on almost any tool chest, Leatherman, swiss army knife, or other pocket mechanic tool to have a flathead screwdriver built in. This is why they're so ubiquitous.