[...] the term 'root', for instance, as a designation for the central part of a word, comes from Hebrew grammar. - Bloomfield (1935), a pag.10
The root plays the same part in primary words as the underlying form (e.g. 'dance', 'lead', 'ride') in secondary words ('dancer', 'leader', 'rider'). - Bloomfield (1935), a pag.240