For the NYPD officer, please see Charley.
The term Charlie was the phonetic letter, used on Earth, for the letter "C." It could also be used as the nickname for people named Charles.
A security code created by Data to lock all command functions of the USS Enterprise-D in 2367, as well as Beverly Crusher's authorization code in 2373, utilized the term "Charlie". (TNG: "Brothers"; Star Trek: First Contact)
Charles Evans was generally called "Charlie" in 2266. (TOS: "Charlie X")
Charles Tucker III was at one point supposed to have the nickname Charlie before the decision was made to use "Trip".