(physics) Any device (such as aradiator) that transfers heat from one fluid to another via a separating wall.
1960 June, “The N.B. Loco. Co. diesel-hydraulic Type "2" locomotive”, inTrains Illustrated, page345:
The transmission oil is cooled in aheat exchanger through which the cooling water is circulated, to assist rapid warming of the engine system and to bring engine and transmission into their true interdependence.
