fundamental frequency vs first harmonic

fundamental frequency and first harmonic both are nouns.

Word	Noun	Adjective	Verb	Adverb
fundamental frequency	Yes	No	No	No
first harmonic	Yes	No	No	No

As nouns, fundamental frequency and first harmonic are synonyms defined as:

fundamental frequency and first harmonic: the lowest tone of a harmonic series

Other synonyms of fundamental frequency include fundamental.

fundamental frequency (noun)	first harmonic (noun)
the lowest tone of a harmonic series	the lowest tone of a harmonic series