Human language is unique in comparison to other forms of communication, such as those used by non-human animals.
Communication systems used by other animals such as bees or non-human primates are closed systems, that consist of a closed number of possible things that can be communicated. Human language is open-ended, meaning that it allows humans to produce an infinite set of utterances from a finite set of elements. This we can do because human language is based on a dual code, where a finite number of meaningless elements (e.g. sounds, letters or gestures) can be combined to form units of meaning (words and sentences). Furthermore the symbols and grammatical rules of any particular language are largely arbitrary, meaning that the system can only be acquired through social interaction. The known systems of communication used by animals, on the other hand, can only express a finite number of utterances that are mostly genetically transmitted.
Human languages also differ from animal communication systems in that they employ grammatical and semantic categories such as noun and verb, or present and past, to express exceedingly complex meanings. Human language is also unique in having the property of recursivity; this is the way in which, for example, a noun phrase to contain another noun phrase (as in "the chimpanzee's lips") or a clause to contain a clause (as in "I think that it's raining").. Human language is also the only known natural communication system that is modality independent, meaning that it can be used not only for communication through one channel or media, but through several - for example spoken language uses the auditive modality, whereas sign languages and writing use the visual modality and braille writing uses the tactile modality.
With regards to the meaning that it may convey and the cognitive operations that it builds on, human language is also unique in being able to refer to abstract concepts and to imagined or hypothetical events, as well as events that took place in the past or may happen in the future. This ability to refer to events that are not at the same time or place as the speech event is called "displacement", and while some animal communication systems can use displacement the degree to which it is used in human language is also considered unique.