First language and Natural language

First language noun - One's native language; the language learned by children and passed from one generation to the next.

Natural language and first language are semantically related. In some cases you can use "Natural language" instead a noun phrase "First language".

Natural language noun - A human written or spoken language used by a community; opposed to e.g. a computer language.

First language and natural language are semantically related. Sometimes you can replace phrase "First language" with "Natural language", this phrases are similar.