Noun
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.
A definition being developed; a tentative definition that can be tailored to create an authoritative definition.
Source: en.wiktionary.orgA perpetual holiday is a good working definition of hell. George Bernard Shaw
A working definition of fathering might be this: fathering is the act of guiding a child to behave in ways that lead to the child's becoming a secure child in full, thus increasing his or her chances of being happy and fruitful as a young adult. Clyde Edgerton