Yet, clearly, no one would say that they speak different languages. Generally, systems of vocal communication are recognized as different languages if they cannot be understood without specific learning by both parties, though the precise limits of mutual intelligibility are hard to draw and belong on a scale rather than on either side of a definite dividing line. Substantially different systems of communication that may impede but do not prevent mutual comprehension are called dialects of a language. In order to describe in detail the actual different speech patterns of individuals, the term idiolect, meaning the speech habits of a single person, has been coined.
3. Normally, people acquire a single language initially "their first language, or mother tongue, the language spoken by their parents or by those with whom they are brought up from infancy. Subsequent "second- languages are learned to different degrees of competence under various conditions, but the majority of the world's population remains largely monolingual. Complete mastery of two languages is designated as bilingualism; in a few special cases "such as upbringing by parents speaking different languages at home "speakers grow up as bilinguals, but ordinarily the learning, to any extent, of a second or other language is an activity superimposed on the prior mastery of one's first language and is a different process intellectually.
4. Language, as described above, is species-specific to man. Other members of the animal kingdom have the ability to communicate, through vocal noises or by other means, but the most important single feature characterizing human language (that is, every individual language), against every known mode of animal communication, is its infinite productivity and creativity. Human beings are unrestricted in what they can talk about; no area of experience is accepted as necessarily incommunicable, though it may be necessary to adapt one's language in order to cope with new discoveries or new modes of thought.