Panthera leo vs big cat

Panthera leo and big cat both are nouns.

Word	Noun	Adjective	Verb	Adverb
Panthera leo	Yes	No	No	No
big cat	Yes	No	No	No

As nouns, big cat is a hypernym of Panthera leo; that is, big cat is a word with a broader meaning than Panthera leo:

Panthera leo: large gregarious predatory feline of Africa and India having a tawny coat with a shaggy mane in the male
big cat: any of several large cats typically able to roar and living in the wild

Other hypernyms of Panthera leo include cat.

Panthera leo (noun)	big cat (noun)
large gregarious predatory feline of Africa and India having a tawny coat with a shaggy mane in the male	any of several large cats typically able to roar and living in the wild