big tree vs Sequoiadendron giganteum

big tree and Sequoiadendron giganteum both are nouns.

Word	Noun	Adjective	Verb	Adverb
big tree	Yes	No	No	No
Sequoiadendron giganteum	Yes	No	No	No

As nouns, big tree and Sequoiadendron giganteum are synonyms defined as:

big tree and Sequoiadendron giganteum: extremely lofty evergreen of southern end of western foothills of Sierra Nevada in California; largest living organism

Other synonyms of big tree include giant sequoia, Sequoia gigantea, Sequoia Wellingtonia, Sierra redwood.

big tree (noun)	Sequoiadendron giganteum (noun)
extremely lofty evergreen of southern end of western foothills of Sierra Nevada in California; largest living organism	extremely lofty evergreen of southern end of western foothills of Sierra Nevada in California; largest living organism