Wiktionary
n. 1 A definition that is chosen for an occasion and may not fully conform with established or authoritative definitions. Not knowing of established definitions would be grounds for selecting or devising a working definition. 2 A definition being developed; a tentative definition that can be tailored to create an authoritative definition.
Usage examples of "working definition".
She was, nevertheless, compelled to concede that an unending future in which an unending parade of waitresses served up unending plates of hash would strike some people as a pretty good working definition of hell, while still others might ask for no finer paradise.
Of course, there are some who would call it a good working definition of an obsession.
And wasn't that a working definition of freedom, unspoiled by European hand, European certainty?