There was a prior discussion about teaching organic chemistry. I will use this topic to show a different notion about teaching ochem. If we were discussing someone playing catch, I argue a description of the ball or the glove would not by itself help you to understand the action of playing catch. If you saw someone playing catch, you could understand the process easily.
Describing a nucleophile is like describing a ball, but the question about what is substitution is like what is playing ball. I prefer to discuss nucleophiles after showing examples (including the bond making and breaking processes or mechanism) of substitution reactions. Then the nucleophile is the group that donated electrons in forming a bond. If you were accustomed to substitution reactions, then you would intuitively what a nucleophile was. What you would learn would be the term nucleophile.
I further argue that if you were familiar with a number of nucleophilic substitution reactions, you would also be familiar with examples of nucleophiles and leaving groups. At this point, a description of "good" nucleophiles would summarize what you might already know. I think of this as "example based learning".
So, look over some nucleophilic substitution reactions. The group donating electrons is the nucleophile. Paradoxically, one of the best nucleophiles is also one of the best leaving groups. That will be like the best ball to catch is also the best one to throw.