A stylus (plural: styli) is a writing utensil. It usually refers to a narrow, elongated staff, similar to a modern ballpoint pen. Many styli are heavily curved to be held more easily.
Styli were first used by the ancient Mesopotamians in order to write in cuneiform. Styli were usually made out of reeds that grew on the sides of the Tigris and Euphrates and down to Egypt where the Egyptians used styli from sliced reeds with sharp points. Cuneiform was entirely based on the "wedge-shaped" mark that the styli made when pushed into a clay tablet, hence the name cuneiform coming from the Latin root cuneus meaning wedge-shaped.
In the sound recording industry, a stylus is a phonograph or gramophone needle used to play back sound on gramophone records, as well as to record the sound indentations on the master record. Today, the term stylus often refers to an input method usually used in PDAs and graphics tablets. In this method, a stylus that secretes no ink touches a touch screen instead of a finger to avoid getting the natural oil from one's hands on the screen, or produces brushstrokes in a computer screen, respectively. Styli are also used with the Nintendo DS handheld gaming device, which has two screens, the bottom one being touch-sensitive.
A stylus may also be used to scribe a recording into smoked foil or glass. In various instruments this method may be used instead of a pen for recording as it has the advantage of being able to operate over a wide temperature range, does not clog or dry prematurely, and has very small friction in comparison to other methods. These characteristics were useful in certain types of early seismographs and in recording barographs used in determining sailplane altitude records.