sand dune vs seif dune

sand dune and seif dune both are nouns.

Word	Noun	Adjective	Verb	Adverb
sand dune	Yes	No	No	No
seif dune	Yes	No	No	No

As nouns, seif dune is a hyponym of sand dune; that is, seif dune is a word with a more specific, narrower meaning than sand dune:

sand dune: a ridge of sand created by the wind; found in deserts or near lakes and oceans
seif dune: a long and tall sand dune with a sharp crest; common in the Sahara

sand dune (noun)	seif dune (noun)
a ridge of sand created by the wind; found in deserts or near lakes and oceans	a long and tall sand dune with a sharp crest; common in the Sahara