Principles and Parameters
Definition - A two-part framework within generative linguistics in which a language's syntax is described according to:
(1) non-variable principles (i.e. rules that all languages must obey) and
(2) variable parameters that for particular languages are either turned on or off.

For example:
(1) That sentences must always have subjects, even if the subject is not overtly pronounced is a principle, and
(2) whether a language is head-initial is a parameter that is either on or off for particular languages.

Etymology -
This framework was formulated by Noam Chomsky and Howard Lasnik.

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