Remaining particles after making coffee or tea
dregs [drɛgz] pl n
1. (Miscellaneous Technologies / Brewing) (Cookery) solid particles that tend to settle at the bottom of some liquids, such as wine or coffee
2. residue or remains
meta: search for the words "coffee OR tea particles remain at the bottom" in Google and you will see the first dictionary result is the TFD entry above! :)
" Sediment " should be the close one ..