root form
/ˈruːt fɔːm/noun
1 Linguistics. A word form consisting of an unmodified root, especially an uninflected verb form.
2 Zoology. A basal or ancestral form of an organism. Now rare.
3 Entomology. A wingless form occurring in some plant lice (especially of the family Phylloxeridae) feeding on roots.
Origin
Mid 19th century; earliest use found in Congregational Magazine. From root + form.