Lewis & Short

sĕrum, i, n. (collat. form sĕrū, acc. to Charis. p. 23 P.) [prob. sibilated from ὀρός].

  1. I. The watery part of curdled milk, whey, Plin. 11, 41, 96, § 239; 28, 9, 33, § 126; Col. 7, 12, 10; Verg. G. 3, 406; Tib. 2, 3, 16; Ov. F. 4, 770.
  2. II. Transf., like ὀρός, of the watery parts, serum, of other things; of resin, Plin. 16, 12, 23, § 60: seminis genitalis, Cat. 80, 8.