Lewis & Short

astĭpŭlātĭo (adst-), ōnis, f. [astipulor]; lit., an assent to or agreement with; hence,

  1. I. An assenting to, affirming the same facts: quā de re exstat etiam Annaei Senecae adstipulatio, Plin. 29, 1, 5, § 10.
  2. II. A modulation of the voice according to the sentiment; Accedit enim vis et proprietas rebus tali adstipulatione, quae nisi adsit, aliud vox, aliud animus ostendat, Quint. 11, 3, 175.