Lewis & Short

ădumbrātĭo, ōnis, f. [adumbro], a sketch in shadow, à la silhouette, a perspective sketch or draft (cf. adumbro).

  1. I. Lit.: scenographia est frontis et laterum abscedentium adumbratio, Vitr. 1, 2.
  2. II. Fig., a sketch, outline: nulla est laus oratoris, cujus in nostris orationibus non sit aliqua, si non perfectio, at conatus tamen atque adumbratio, * Cic. Or. 29.
    Hence,
    1. B. A false show, the semblance of a thing, pretence: insidiosa beneficii adumbratio, Val. Max. 7, 3, 8; cf. Cic. Off. 1, 14, 44.