leatherback vs sea turtle

leatherback and sea turtle both are nouns.

Word	Noun	Adjective	Verb	Adverb
leatherback	Yes	No	No	No
sea turtle	Yes	No	No	No

As nouns, sea turtle is a hypernym of leatherback; that is, sea turtle is a word with a broader meaning than leatherback:

leatherback: wide-ranging marine turtle with flexible leathery carapace; largest living turtle
sea turtle: any of various large turtles with limbs modified into flippers; widely distributed in warm seas

Other hypernyms of leatherback include marine turtle.

leatherback (noun)	sea turtle (noun)
wide-ranging marine turtle with flexible leathery carapace; largest living turtle	any of various large turtles with limbs modified into flippers; widely distributed in warm seas