impromptu

an extemporaneous speech or remark

• “a witty impromptu must not sound premeditated”

a short musical passage that seems to have been made spontaneously without advance preparation

with little or no preparation or forethought

• “his ad-lib comments showed poor judgment”
• “an extemporaneous piano recital”
• “an extemporary lecture”
• “an extempore skit”
• “an impromptu speech”
• “offhand excuses”
• “trying to sound offhanded and reassuring”
• “an off-the-cuff toast”
• “a few unrehearsed comments”

without advance preparation

• “he spoke ad lib”