m-w.com - impromptu : improvised

impromptu   \im-PROMP-too\   adjective
        1 : made, done, or formed on or as if on the spur of the moment : improvised
   

Example sentence:
         When we dropped by unexpectedly, Aunt Dinah threw together an impromptu dinner from the odds and ends in her refrigerator.

Did you know?
         If you think that "impromptu" looks like a relative of "prompt," you're right; both are ultimately derived from the Latin "promere," meaning "to bring forth, take out." "Impromptu" was borrowed from French, where it meant "extemporaneously," but French speakers picked it up from the Latin phrase "in promptu," a "promere" descendant meaning "in readiness" or "at hand." There is also another, much rarer descendant of "promere" in English -- the noun "promptuary," meaning "a book of ready reference."