What is the key on a pie chart?

Here are two explanations that are both correct. Use whichever works for you:

1. Its basically the bit, usually in the corner, that explains what each colour (or whatever is being used) on the pie chart represents.
2. So, you might have a pie chart that is divided into sections, with each section having a different colour. The key is often a short list of small squares that are in all the colours on the pie chart. Besides these squares will be the thing that their colour represents on the pie chart.