1840s vs decade

1840s and decade both are nouns.

Word	Noun	Adjective	Verb	Adverb
1840s	Yes	No	No	No
decade	Yes	No	No	No

As nouns, decade is a hypernym of 1840s; that is, decade is a word with a broader meaning than 1840s:

Other hypernyms of 1840s include decennary, decennium.

1840s (noun)	decade (noun)
the decade from 1840 to 1849	the cardinal number that is the sum of nine and one; the base of the decimal system
	a period of 10 years