What counts as nudes?

Im on another app and there are these girls who sent me pics of them in their underwear and i told them they look nice but i dont do nudes and they told it's not a nude they're just panties and then ask if i would do the same and i told them again i dont do nudes and i don't know matbe im a prude or something but it's weird. we show ourselves in bikinis but in underwear its a private thing. I don't know i guess im too prudey