A box spring is a frame containing rows of coiled springs that is used to provide support for a mattress. You can also use box springs to refer to the springs themselves.
box spring in British English
noun
a coiled spring contained in a boxlike frame, used as a base for mattresses, chairs, etc
box spring in American English
US
a bedspring consisting of a boxlike, cloth-enclosed frame containing rows of coil springs fastened to the frame
Examples of 'box spring' in a sentence
box spring
Beneath her, beneath the mattress and the box spring, under the dust ruffle, there was exactly nothing.