Lewis & Short

mīlĭfŏlĭum and millefŏlĭum, ii, n. [mille-folium], a plant, milfoil, yarrow; form milifolium: myriophyllus, quod nostri milifolium vocant, caulis est tener, similis feniculi, Plin. 24, 16, 95, § 152 Jan. (al. millefolium); used in medicine, id. 24, 19, 116, § 176 al.
Form millefolium, Plin. 25, 5, 19, § 42 Jan. (al. millefolia).