extempore
Concise Oxford English Dictionary © 2008 Oxford University Press:
extempore /ɪkˈstɛmp(ə)ri, ɛk-/
▶adjective & adverb spoken or done without preparation.
– origin C16: from L. ex tempore ‘on the spur of the moment’ (lit. ‘out of the time’).
'extempore' also found in these Oxford entries:

