An individual language (usually simply denoted by a language i.e. used as a countable noun) is a specific use of a codified subset of language (used as an uncountable noun). English is an example of an individual language.
Deciding the boundaries between languages is somewhat tricky. For example, in the case of English, there is dispute over whether Scots is a dialect of English or a language in its own right. Other cases are more clear cut.