The art of making a good definition is to find the right balance between too much and too little and to state only the absolutely necessary conditions. A too restrictive definition has fewer chances to be adopted than a broader one, and a too broad definition risks losing its defining function.
A proper definition defines exactly what is supposed to define and nothing else. A good definition gives an if-and-only-if condition for when an object or a term satisfies the definition. It is conclusive and exclusive. A good definition should also involve simpler terms than what we are defining.
Sometimes, we are compelled to use working definitions. A working definition is a definition we choose for an occasion and may not fully conform to the established definition. It is a definition at a stage of being developed.—a tentative definition that in due time may turn into an established definition.
The word or group of words that is to be defined is called the definiendum, and the word, group of words or action that defines it is called the definiens.
Definitions should avoid circularity, it must not include the definiendum. For example, “Aggression is used to protect the animal through the use of aggressive displays (…)” is a circular definition that does not add anything to the term we are defining.