raft vs understructure

raft and understructure both are nouns.

Word	Noun	Adjective	Verb	Adverb
raft	Yes	No	Yes	No
understructure	Yes	No	No	No

As nouns, understructure is a hypernym of raft; that is, understructure is a word with a broader meaning than raft:

raft: a foundation (usually on soft ground) consisting of an extended layer of reinforced concrete
understructure: lowest support of a structure

Other hypernyms of raft include base, foot, foundation, fundament, groundwork, substructure.

raft (noun)	understructure (noun)
a flat float (usually made of logs or planks) that can be used for transport or as a platform for swimmers	lowest support of a structure
a foundation (usually on soft ground) consisting of an extended layer of reinforced concrete
(often followed by `of') a large number or amount or extent