Littering was the act of disposing of garbage in an incorrect manner.
In 2373, Benjamin Sisko warned Quark that if he so much as littered on the Promenade, he'd nail him to the wall. (DS9: "Business as Usual")
In 2375 Ezri found out her previous host Jadzia ordered takeout for Martok's birthday and the crates of gagh were in the cargo bay. Ezri, who had not gotten used to gagh, tried to dump it out the airlock but was stopped by Odo, who told her Bajoran environmental laws prohibited it and she would be littering. (DS9: "Prodigal Daughter")