Universal translator
The universal translator is a device used to decipher and interpret alien languages into the native language of the user.
On Earth, the universal translator was invented shortly before 2151, and was still experimental at the time of the launch of Enterprise. This version of the UT could be used for ship-to-ship communications only, and face-to-face communication or off-world missions required the use of a skilled linguist - in Enterprise's case, Hoshi Sato - notably in situations where reading alien languages on the control panels, hatches, and displays were involved.
By 2267, the UT had been miniaturized into a handheld, wand-like device. Captain James T. Kirk of the USS Enterprise used one to communicate with the alien known as the Companion in the Gamma Canaris system. Responding to Zefram Cochrane's question about the theory of operation, Kirk explained that there are certain universal ideas and concepts common to all intelligent life. The translator compares the frequencies of brainwave patterns, selects those ideas it recognizes, and provides the necessary grammar. Kirk further explained that the device speaks with a voice, or the approximation of one, that corresponds to the identity concepts it recognizes. The Companion was revealed to be female because the universal translator detected this facet of its identity from its brainwave patterns, and assigned it a female voice.
In the 24th century, UT technology had advanced to where it could be integrated into the combadges worn by Starfleet personnel.
The universal translator fails from time to time, as with the Tamarians, whose metaphorical language defeated the mechanism completely. It was also incapable of deciphering the nuances of Dominionese.
Another language the UT had trouble deciphering was that of the Skrreeans. Several hours were required to record and analyze the language before a proper translation form was created.
Ferengi translators were small devices inserted into the ear. They were easily disrupted by radiation.
Cardassian universal translators have to be adjusted in order to properly translate the Breen language.