An alias or code name was a name chosen by an individual for the purpose of concealing or obscuring their true identity.
While chasing a fugitive Barry Waddle, aka a future version of Arne Darvin, who was posing as a Human in both 2268 (Darvin) and 2373 (Waddle), Jadzia Dax used make-up to cover her Trill spots, thereby posing as a Human or other species of similar appearance. Odo, whose species was unknown to the Alpha Quadrant at the time, was also pretending to be something other than what he was. (DS9: "Trials and Tribble-ations")
A Romulan identity was assumed by James T. Kirk for the purposes of seizing a cloaking device onboard a Romulan flagship later that year. Doctor Leonard McCoy altered his physical appearance to enable him to blend in following the captain's apparent death caused by Spock's use of the Vulcan death grip. When confronted by a guard, he claimed to have escaped the USS Enterprise with important information for Sub-commander Tal. He subsequently subdued two officers and beamed off the flagship. Later, his true identity was revealed to the commander when she was beamed aboard the Enterprise. (TOS: "The Enterprise Incident")