Language

Language is a systematized form of communication that most commonly occurs in spoken, written, signed/gestural, and thought (telepathic) forms, although other methods of communication have been observed in the known galaxy. The use of language is one of the defining features of sentience; though non-sentient species may exhibit some of the properties of language in their communications, the possession of all of these characteristics is the domain of sentients.

General features of natural language
Languages are set apart from random signals and non-sentient communication in that set relationships between the various components are determined by a systematized grammar, a set of rules by which that language operates. Even in the most extreme examples of telepathic language (i.e. that of the Cairn), which are otherwise devoid of a recognizable set of symbols, the images, emotions, and concepts conveyed are bound together by the logic of the situations depicted such that the meaning intended by one interlocutor can be accurately deduced by the other.

The symbols used in languages--aside, perhaps, for the direct images used by the Cairn language and components of certain other telepathic forms of communications--are arbitrary in nature. That is to say that while a symbol (a morpheme or word, for instance) may be assigned one meaning in one language, it need not carry that meaning in another language. For instance, the name "Bena" (daughter of Ferengi Grand Nagus Rom and his Bajoran wife Leeta) means "joy" in Bajoran, but "underflooring" in Ferengi. (DS9 novel: Ferenginar: Satisfaction is Not Guaranteed)