task-wheel, n. 1. A circular device of indeterminate origin, bearing inscribed grooves of varying depth, used to measure the specific gravity of a weekday. Cf. millstone, helm, calendar. 2. (Obsolete.) A penitential instrument in which each spoke represents one act of will. Distinguished from the prayer wheel by the absence of prayer. 3. (Botanical, rare.) The cross-section of a eucalyptus trunk, in which the growth rings are sometimes mistaken for obligations. Under magnification, the thirty-sixth ring is always green.