A stylus (plural styli) is a writing instrument. It ususally refers to a narrow, elongated staff, similar to a modern ball-point pen. Many styli (plural of stylus) are slightly curved to be held more easily.
"Stylus" is also the term usually used by those in the sound recording industry for what is more commonly called a phonograph or gramophone "needle", used to play back sound on analogue disc 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 PDAss. 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.