An alias or code name was a name chosen by an individual for the purpose of concealing or obscuring their true identity.
"Gary Seven" was a code name. (TOS: "Assignment: Earth")
The con artist posing as Ardra was known by twenty-three aliases in the area of Ventax II alone, a fact her crew divulged to Jean-Luc Picard in 2367. (TNG: "Devil's Due")
In 2371, Quark discovered that his mother had been conducting business transactions across the Ferengi Alliance using dozens of different aliases. (DS9: "Family Business")