stain 1 /steɪn/ ●○○ verb 1 [intransitive, transitive] to accidentally make a mark on something, especially one that cannot be removed, or to be marked in this way
Be careful you don’t stain the carpet.
This tablecloth stains very easily.
Her fingers were stained yellow from years of smoking.
a cowboy hat stained with dust and sweat
There are some stains on your pillow. It could be your child poured water or milk or sweat on the pillow, but you don't know what substance stained it.
Do you say "the pillow is stained with some substance"?
or do you use a better word instead of using "substance"?