An electric circuit connected at the end of a bus to hold it at a predetermined signal level when it is not active, and also to ensure impedance matching and thus avoid unwanted reflections of signals. It is often available as a single package for mounting onto a printed circuit board.
It is important to ensure that the electric impedance of a bus carrying high-frequency signals does not have any abrupt changes. If the ends of the conductors are not terminated, the signals see an almost infinite impedance and are reflected back along the conductor. A fast switching circuit connected to an unterminated bus could detect both the signal and the reflection and so give rise to errors.