There is a common "myth" throughout fandom, reinforced by some portrayals in the novels, that Uhura is principally a linguist. Never, though, is she shown to have a proclivity for languages. She knows English and her native tongue of Swahili (much like Chekov knows Russian), but she doesn't seem to know Klingon (TUC) or any other language. Nor does she try to help out when linguistics would be useful in deciphering alien communications (TVH). Linguistics is a science, but she instead wears the red uniform of Engineering and Support Services (and, yes, she wore a gold Command uniform in "The Corbomite Maneuver").
As she is portrayed in TOS (and the films), I'd argue, she is a talented engineer who specializes mainly in communications systems but also knows a number of other systems, like the navigation console ("Balance of Terror") and science station ("The Galileo Seven," "Whom Gods Destroy"). For example, she was the one who identified the energy pulsations in "Spock's Brain" that led the crew to the correct planet. And she took command in TAS ("Bem," "The Lorelei Signal"), fitting that she was once in the Command division. Later, in TSFS, she took a post manning a transporter room, something she must have been qualified to do (even if Starfleet basically just let her volunteer to do whatever she wanted). Also Uhura was one of only 20 officers chosen by M-5 to operate the ship during its war games ("The Ultimate Computer").
Probably the best example is "Who Mourns for Adonais?", where she rewires the entire communications system to break through the Apollo's interference. Spock actually praises her work to wire a bypass circuit, and he even says that he can think of "no one better equipped" to do this engineering feat. She also did engineering work on the communication console in "This Side of Paradise."
The 1967 Writers' Guide for Star Trek's second season described her as: "Communications officer Uhura was born in the United States of Africa. Quick and intelligent, she is a highly efficient officer and expert in all ships' systems related to communications." Nothing about linguistics or languages.
The 1977 Writers'/Directors' Guide for Star Trek: Phase II goes even further: "Uhura was born in the African Confederacy. Quick and intelligent, she is a highly efficient officer. Her understanding of the ship's computer systems is second only to the Vulcan Science Officer, and expert in all ships systems relating to communications."
Uhura appears to be a talented engineer, not a linguist, even though interpretations of her character tend to favor the latter. Presumably this has to do with wanting to see her as "more" than "just" a future telephone operator. But clearly she was more than a secretary, although she's an engineer and communications systems specialist, not a linguist, given her portrayal onscreen.