lighter vs pontoon

lighter and pontoon both are nouns.

Word	Noun	Adjective	Verb	Adverb
lighter	Yes	No	Yes	No
pontoon	Yes	No	No	No

As nouns, pontoon is a hyponym of lighter; that is, pontoon is a word with a more specific, narrower meaning than lighter:

lighter: a flatbottom boat for carrying heavy loads (especially on canals)
pontoon: (nautical) a floating structure (as a flat-bottomed boat) that serves as a dock or to support a bridge

Other hyponyms of lighter include dredger, houseboat, scow, Norfolk wherry, wherry.

lighter (noun)	pontoon (noun)
a device for lighting or igniting fuel or charges or fires	a float supporting a seaplane
a flatbottom boat for carrying heavy loads (especially on canals)	(nautical) a floating structure (as a flat-bottomed boat) that serves as a dock or to support a bridge
a substance used to ignite or kindle a fire

lighter (verb)	pontoon (verb)
transport in a flatbottom boat