duke vs Lord

duke and Lord both are nouns.

Word	Noun	Adjective	Verb	Adverb
duke	Yes	No	No	No
Lord	Yes	No	No	No

As nouns, Lord is a hypernym of duke; that is, Lord is a word with a broader meaning than duke:

Other hypernyms of duke include noble, nobleman.

duke (noun)	Lord (noun)
a British peer of the highest rank	a titled peer of the realm
a nobleman (in various countries) of high rank	terms referring to the Judeo-Christian God