I can't help but feel stupid for asking this, but what does it mean to "define" a plugin?
To develop one. However, there are templates that make it easier, so you just customise the template and write your plugin logic.
It sounds like bad wording somewhere - where does it say "define a plugin?
It said it on a few sites I was on, but I've lost the links. Also do you know where I can get the template you mentioned?