What is the definition of 'agon' as in hexAGON and pentAGON?

My son has been asked to do a search on this - he is in year 1 (aged 6) at school, and he keeps asking the teachers questions that they don't know the answers to so they tell him to search it on the internet. We've tried a google search and wikipedia with no luck. Thanks for trying to answer this one, you'll make my boys day :-)