lingua franca vs koine

lingua franca and koine both are nouns.

Word	Noun	Adjective	Verb	Adverb
lingua franca	Yes	No	No	No
koine	Yes	No	No	No

As nouns, lingua franca and koine are synonyms defined as:

lingua franca and koine: a common language used by speakers of different languages

Other synonyms of lingua franca include interlanguage.

lingua franca (noun)	koine (noun)
a common language used by speakers of different languages	a common language used by speakers of different languages