Formants are different than more common combinations of frequency components, such as harmonic series.
British Dictionary definitions for harmonic series
(maths) a series whose terms are in harmonic progression, as in 1 + 1/2 + 1/3 + …
(acoustics) the series of tones with frequencies strictly related to one another and to the fundamental tone, as obtained by touching lightly the node points of a string while playing it. Its most important application is in the playing of brass instruments