【Extemporaneous】 , 【extempore】 ,  【extemporary】 ,  【improvised】 ,  【impromptu】 ,  【offhand】 ,  【unpremeditated】  mean composed, concocted, devised, or done at the moment rather than beforehand.

【Extemporaneous】 ,  【extempore】 , and  【extemporary】 in their more general applications stress something made necessary by the occasion or situation and may suggest sketchiness or crudity in the thing modified.

The terms may retain this value when applied to modes of expression or to persons as sources of expression, but more usually, and in technical context routinely, they imply a well thought-out plan or outline that is given spontaneity in presentation by fresh, unstudied choice of words.

【Improvised】 stresses the absence of foreknowledge of what is to be accomplished and therefore the composing, concocting, devising, or constructing of something without advance thought or preparation and often without the necessary tools, instruments, or other equipment.

【Impromptu】  stresses the immediate response to a need or suggestion and the spontaneous character of what is composed or concocted on the spur of the moment; thus, an  【impromptu】  speech is one prepared at a moment’s notice and delivered without notes or a preconceived plan; an  【impromptu】 meal is one prepared from what is available usually at an unusual time or for an unexpected number of people .

【Offhand】 , both as adjective and adverb, carries so much stronger an implication of casualness, carelessness, or indifference than any of the preceding terms that at times it loses its suggestion of an 【impromptu】 character and means little more than curt or brusque.

【Unpremeditated】  emphasizes less strongly than  【extemporaneous】  and  【impromptu】 the immediate stimulus of an occasion, but it usually suggests some strong, often suddenly provoked emotion which drives one to action.