Palmus, palmi, m. g. Bud. Plin. A measure of an hand breadth.Palmus maior.Twelue fingers or a spanne.Palmus minor.Halfe a foote.Palmum neutro genere etiam dicitur. Vitruuius.
Lewis and Short: Latin dictionary
palmus, i, m. [1. palma], the palm of the hand. I.Lit.: manūs palmum, Vitr. 2, 3: Graeci antiqui doron palmum vocabant: et ideo dora munera, quia manu darentur, Plin. 35, 14, 49, 171.—II.Transf., as a measure of length, a span, or twelvedigits, Varr. R. R. 3, 7: amomum frutex myrtuosus, palmi altitudine, Plin. 12, 13, 28, 48: minor, a smaller measure, a palm, or four digits, acc. to Vitr. 3, 1, 5.
