Extempore, adv.
Etymology: [L. ex out + tempus, temporis, time. See Temporal.]
Without previous study or meditation; without preparation; on the spur of the moment; suddenly; extemporaneously; as, to write or speak extempore. Shak. -- a.
Done or performed extempore. "Extempore dissertation." Addison. "Extempore poetry." Dryden. -- n.
Speaking or writing done extempore. [Obs.] Bp. Fell.
Shades of meaning
Fernald has no entry of its own for extempore; he lists it under extemporaneous.
He lists it without comment; the entries above explain its neighbors.