Extempore
Off the cuff, improvised.
What it means
Extempore comes from Latin and means “out of the moment.” What is meant is a performance without preparation: the performer invents the music at the moment of playing.
The term is largely synonymous with improvisation. Where a distinction is made, extempore stresses more strongly the spontaneity of the occasion — something is inserted, added, invented off the cuff — whereas improvisation designates the process in general.
Both are to be distinguished from sight-reading: there an existing score is read without preparation, here something is invented without a model. The term is likewise to be kept apart from ad libitum, which grants a freedom within a notated work.
In the notation
As an instruction the word seldom appears. Where a passage is to be invented freely, this is rather indicated by markings such as cadenza, a fermata over a rest, or an empty bar with a corresponding note. In figured-bass parts, the free filling-in of the texture is presupposed anyway and is not designated separately.
Musical use
Inventing off the cuff was part of the musician's craft for centuries. Organists improvised preludes and postludes, soloists their cadenzas, continuo players their texture. In the course of the nineteenth century this practice recedes in art music, while it remains alive in jazz, folk music, and church music. In historically informed performance practice it has been practised again for some decades.
The expression is also used in the theatre, where it means the improvised addition of an actor. In music it designates playing off the cuff, that is, without preparation. Today improvisation is usually said instead; extempore sounds old-fashioned and is used rather in the sense of a spontaneous idea.
Performance practice
Improvising does not mean being without preparation: whoever has a stock of turns, cadence models, and figures invents more freely. Before the first note, decide on key, metre, and length. Think in phrases and plan a goal point. And keep the pulse — a halting course betrays uncertainty faster than a wrong note.