A stylus (plural, styli) was a tool to operate log pads and hand-held computer interfaces commonly used throughout the Federation and many other civilizations throughout the Milky Way Galaxy, for the easy access and sharing of data. Gilden helped James T. Kirk open a magnetic lock with two styli from his log pad. (TOS novel: Crossroad)
Peter Kirk, used one with a Message Pad to write and to send a message to James T. Kirk after his talk with about Keep Earth Human League, to Commander Gordon Twelvetrees, in the Federation Security Office. (TOS novel: Sarek)
In 2270, James Kirk, in combat with Gary Seven, used a stylus to stab out the Aegis agent's eye. (TOS - Year Five - The Wine-Dark Deep comic: "Issue 12")