Roget's Thesaurus of English Words and Phrases by Peter Mark Roget

612. Impulse — N. impulse, sudden thought; impromptu, improvisation;

inspiration, flash, spurt. improvisatore†; creature of impulse. V. flash on the mind. say what comes uppermost; improvise, extemporize. Adj. extemporaneous, impulsive, indeliberate†; snap; improvised, improvisate†, improvisatory†; unpremeditated, unmeditated; improvise; unprompted, unguided; natural, unguarded; spontaneous &c (voluntary) 600; instinctive &c 601. Adv. extempore, extemporaneously; offhand, impromptu, a limproviste [Fr.]; improviso†; on the spur of the moment, on the spur of the occasion.