plasto

A modern root that does not decompose. Plasto is one of the youngest words in the core vocabulary, and it behaves like any other simple noun: root plus -o. There is no verb plasti for it to be the noun of, and that is worth saying because so many Esperanto nouns are exactly that: sento is the noun of senti, movo of movi, kresko of kreski. The instinct to hunt for the verb behind an -o word is a good one and it simply comes up empty here, so take the root whole.

The adjective is plasta. A plastic bag is a plasta sako, a plastic chair a plasta seĝo, and that is the ordinary way to say the material something is made of. You will also see el plasto - made out of plastic - which is what prepositions offer when you want the material named as a noun instead: botelo el plasto, exactly parallel to domo el ligno, a wooden house.

Watch out for plastiko. Older dictionaries and some writers use plastiko for the material. Today plastiko more usually means the plastic ARTS - sculpture and modelling, the plastikaj artoj - and plasto is the substance, which is the split modern usage has settled on. The word for a plastic surgeon is plastika kirurgo, on the art side of the line, which tells you the two are still related and not interchangeable. If you want to be understood everywhere, say plasto for the stuff in the sea.