Lewis & Short

sēmĭ-vŏcālĭs, e, adj.

  1. I. Half-sounding, half-talking, semi-vocal: instrumentum rusticum, i. e. cattle (distinguished from vocale, slaves, and mutum, i. e. carts), Varr. R. R. 1, 17, 1.
    In like manner: signum militare, i. e. horns, trumpets, etc. (distinguished from vocale, words, and mutum, banners), Veg. Mil. 3, 5.
  2. II. In gram., as subst., a semi-vowel (of which there were, acc. to the old grammarians, the foll. seven: f, l, m, n, r, s, x), Quint. 1, 4, 6; 1, 7, 14; Prisc. p. 540 sq. P.; 564 ib. al.