A root is a contentful °morph (i.e. a morph denoting an °action, an °object or a °property) that can occur as part of a °free form without another contentful morph.
The definition was first given in Haspelmath (2023: 287) and is discussed in detail in Haspelmath (2025). ❡
Other definitions of root make reference to word, e.g. "the unanalyzable core of a word", but since word cannot be easily defined, it is better to define root in terms of the most important meanings. Including other meanings is not necessary, because these never behave in a special way.