Lewis & Short

dēsertĭo, ōnis, f. [2. desero], a forsaking, deserting (rare; perh. not ante-Aug.).
Milit. t. t. (acc. to 2. desero, no. I. B.), desertion: peccatum desertionis, Dig. 49, 16, 3, § 6; 5, § 2 al.