From UBC Wiki

One or more conditions that are necessary but not jointly sufficient for some state of affairs obtaining.


A necessary but not sufficient condition: It is necessary to go to school to be a student but such a condition is not sufficient because a teacher goes to school as well.

In order to get an A in this class it is necessary to write the final exam (because it would be an automatic 25% deduction if one didn't write it), but writing the final exam is not sufficient to get an A in this class (because one might get a low grade on the exam, or fail to complete some other aspect of coursework).