Lewis & Short

pustŭla, ae, f. [pus; cf. pusula].

  1. I. Upon the skin, a blister, pimple, pustule: eruptiones quaedam pustularum, et ulcuscula, Sen. Ep. 72, 6; Plin. 20, 22, 87, § 238; 20, 13, 51, § 141 (pusula in both cases, Jahn); Tib. 2, 3, 9 (passula, M.); Mart. 11, 98.
  2. II. Upon other things, a bubble, blister, e. g. in boiling water, in lime, on earthen-ware, etc., Vitr. 7, 2; Plin. 20, 18, 86, § 203 (pusula, Jahn); 37, 7, 26, § 98 (pusulis, Jahn); Mart. 8, 51, 6.