Sometimes, when someone asks you for a "definition," he/she bugs you because you either "defined" when you should have "described" the subject in question or vice versa.
What does this mean?
Firstly, there are more meanings of define than 'give the exact meaning of a word / lexeme / polyseme'. See at http://www.thefreedictionary.com/define , for instance.
Secondly if we take definition 1b [a] given by the AHD:
we are obviously required to make an attempt to characterise (pin down) the referent. Where the dividing line between 'insufficient details to characterise the referent' and 'a well-defined referent' (ie what qualities are basic? can we differentiate between close relatives using just these tests?) occurs in individual cases is probably a question that must usually be addressed to biologists, mathematicians, medics... rather than linguists.
To define is to delineate a specific meaning of a word. To describe is to provide useful attributes of something represented by that word.
For example, it is hard to define "god" but easy to define "car."
Definition of cell: "the smallest unit of life" or something like that.
Description of cell: "it has ...; it is ..."