Wiktionary
n. (context semantics English) A definition in which a new or currently-existing term is given a new meaning for the purposes of argument or discussion in a given context.
WordNet
n. a definition that is stipulated by someone and that is not a standard usage
Wikipedia
A stipulative definition is a type of definition in which a new or currently-existing term is given a specific meaning for the purposes of argument or discussion in a given context. When the term already exists, this definition may, but does not necessarily, contradict the dictionary ( lexical) definition of the term. Because of this, a stipulative definition cannot be "correct" or "incorrect"; it can only differ from other definitions, but it can be useful for its intended purpose.
For example, in the riddle of induction by Nelson Goodman, " grue" was stipulated to be "a property of an object that makes it appear green if observed before some future time t, and blue if observed afterward." "Grue" has no meaning in standard English; therefore, Goodman created the new term and gave it a stipulative definition.
A governmental example is from the USNRC Final Safety Culture Policy Statement, which defines "nuclear safety culture" as “the core values and behaviors resulting from a collective commitment by leaders and individuals to emphasize safety over competing goals to ensure protection of people and the environment.” By this definition there can never be bad, poor, weak, or otherwise problematic nuclear safety culture. It is, by definition, always good.