1. Said or done without having been planned or written in advance; SYN. spontaneous, unwritten.
2. With little or no preparation or forethought; SYN. extemporaneous, extemporary, extempore, offhand, offhanded, off-the-cuff, unrehearsed.

Without advance preparation; SYN. ad libitum, spontaneously, impromptu.

Remark made spontaneously without prior preparation

1. To speak without a prepared text.
2. (Theater) To improvise new lines when one has forgotten one's part.