A formant is a locally strengthened cluster of adjacent harmonics

Evidence

As we know, a periodic sound can be analyzed into harmonically spaced partials. In principle, the strength of each of the partials is independently controllable. In MAX Partials, we have complete control of 50 of them, including their phases. If a group or cluster of adjacent partials is stronger than surrounding partials, that group is called a formant. — Eric J. Heller, Why You Hear What You Hear- An…, 1669

Categorization

• DEFINE: Gives a precise operational definition of ‘formant’ as a cluster of adjacent partials stronger than neighbors.