adj. Not prepared beforehand; unpremeditated; extemporaneous.
the GNU version of the Collaborative International Dictionary of English
adj. Not prepared or mediated beforehand; extemporaneous.
The Century Dictionary and Cyclopedia
Not studied or prepared beforehand; impromptu; extemporaneous: as, “improviso translation,”
Word Usage
"Terror et metus maxime ex improviso accedentes ita animum commovent, ut spiritus nunquam recuperent, gravioremque melancholiam terror facit, quam quae ab interna causa fit."