Lewis & Short

pĭāmentum, i, n. [pio],

  1. I. a means of expiation, an atoning sacrifice (perh. not anteAug.): terrae, Plin. 25, 9, 59, § 107; 25, 4, 11, § 30; cf.: piamenta, quibus utitur (in) expiando, Fest. s. v. piatrix, p. 213 Müll.; p. 238 ib.
  2. II. Trop., Sen. Cons. ad Helv. 16; Plin. 37, 1, 2, § 3.