Is "liner" a word?

Yes, liner is a valid English word, common noun found in dictionaries, used widely in everyday language, with multiple meanings and recognized in Scrabble and other word games. Liner is noun meaning material used as lining or inner protective layer, such as bin liner, pond liner, panty liner, eyeliner, and clothing liner, and also meaning large passenger ship called ocean liner. It is used as common word in retail, construction, cosmetics, shipping, and engineering. Liner appears in major dictionaries like Oxford, Merriam Webster, and Cambridge with definitions covering inner layer and ship. It can be used as standalone noun, as in trash can liner, and as combining form in words like eye liner, lip liner, and air liner. So liner is definitely a word, legitimate, with plural liners, used in sentences like I need to buy bin liners or The ocean liner crossed Atlantic, and it is not slang but standard English vocabulary with long history of usage.