The term natural language is used to distinguish languages spoken by humans for general-purpose communication from constructs such as computer-programming languages or the "languages" used in the study of formal logic, especially mathematical logic. In the philosophy of language, the term ordinary language is sometimes used as synonymous with natural (as opposed to mathematical or logical) language. Natural language is also considered a field of weak artificial intelligence.
Additionally, the indigenous signed languages of the world merit inclusion as natural languages owing to extensive linguistic analysis in the latter 20th century confirming their unique and consistent grammar, syntax, rules and visual logic dramatically unlike the spoken languages of the nations or geographic regions in which they arose. American, French, and British Sign Languages are the best documented examples in the literature.
Constructed languages such as Esperanto that have evolved to the point of having native speakers may also be considered natural languages. (There are estimated to be 200-2000 native speakers of Esperanto; the number of persons fluent in Esperanto is much larger.)
Natural Languages are deemed to be unsuitable for programming languages simply because they have a vast vocabulary that can be deemed infinite, complex grammatical rules and a sense of ambiguity surrounding them. Take English and French, for example. It takes many years to completely master a language, and this would have been a waste of time when dealing with computing - learning a simple yet efficient embedded language is deemed much easier.