The French term is laverie automatique.
I can't find a mention of a laundromat on the Paris airports website. I expect that many hotels have a laundry service, but only an expensive one where they wash things for you. Only hostels tend to have a washing machine you can use, and even that may not be the case in an urban environment: they might expect you to go to a laundromat.
Searching for laundromats on Google Maps, you can see that they are in the towns near the airport, none in the airport itself. I would expect Google Maps to find these businesses. So there's no walkable option. You'll have to take a bus or train. By train, there's one not far from Sevran-Beaudottes, and one very close to Aulnay-sous-Bois. By bus, I don't know what's the most convenient. Maybe line 95-2 to Mairie de Le Thillay, there's a laundromat just down the street.
Note that almost all laundromats are only open during the day, and coins may be required for payment.