Lewis & Short

vŏla, ae, f., the hollow of the hand, the palm, or (acc. to Fest. p. 370 Müll.), of the foot, the sole, Plin. 11, 45, 105, § 204; Prud. Apoth. 927.
Prov.: nec vola nec vestigium apparet or exstat, i. e. not the slightest trace, Varr. ap. Non. p. 416, 19 and 22.