Saint Martin's summer vs Indian summer

Saint Martin's summer and Indian summer both are nouns.

Word	Noun	Adjective	Verb	Adverb
Saint Martin's summer	Yes	No	No	No
Indian summer	Yes	No	No	No

As nouns, Saint Martin's summer and Indian summer are synonyms defined as:

Saint Martin's summer and Indian summer: a period of unusually warm weather in the autumn

Saint Martin's summer (noun)	Indian summer (noun)
a period of unusually warm weather in the autumn	a period of unusually warm weather in the autumn