A Crystal Filter is a special form of quartz crystal used in electronics systems, in particular communications devices. It provides a very precisely defined centre frequency and very steep bandpass characteristics, that is a very high Q factor - far higher than can be obtained with conventional lumped circuits.
A crystal filter is very often found in the IF stages of high-quality radio receivers. Cheaper sets may use ceramic filters (which also exploit the piezoelectric effect), or tuned circuits. The use of a fixed IF frequency allows a crystal filter to be used because it has a very precise fixed frequency.