Philosophers often want to define terms in very clear ways. One standard that we will use in this class is requiring that there be a definition that gives both (1) necessary conditions and (2) sufficient conditions.
Necessary conditions are conditions or characteristics that must be met in order for the meaning of the term to be satisfied. e.g. For the word nun, it is a necessary condition that one must be a female.
Sufficient conditions are conditions or characteristics that when satisfied, they are sufficient, or adequate to fulfill the meaning of the term. e.g. For nun, one must be a woman who has joined a Catholic religious order.
A definition that gives both necessary and sufficient conditions is often a goal for philosophers
For more on necessary and sufficient conditions, Wikipedia's first few paragraphs on the topics are a good start: