Wiktionary
n. A mattress protector.
WordNet
n. a protective pad over a mattress to protect it
Wikipedia
A mattress pad, mattress topper, or underpad is designed to lie atop a mattress. Made primarily from foam, its function is to provide a layer of comfort and warmth between the user and the mattress. It is of particular benefit when the existing mattress is worn or uncomfortable. Toppers quite often have a contoured profile to facilitate airflow. Visco-elastic foam toppers are regularly used in medical institutions because of their pressure relieving properties. The foam used in the construction of these memory foam toppers reacts to both pressure and temperature. This, combined with its greater density, are properties that give them their characteristic "weightlessness" sensation.
An electric mattress pad is made of heavy fabric with heating filaments woven into it. In the UK and Commonwealth countries this is called an " electric blanket".
Usage examples of "mattress pad".
She lost herself for a few moments then, and when she came back she was pistoning her legs rapidly up and down, pushing the coverlet, the sheets, and the mattress pad down to the foot of the bed.
A couple more meters of the stuff were earmarked for childbirth supplies, and there was enough left to make a single narrow mattress pad for each bed.
The bed had been stripped except for the mattress pad, but there were blankets stacked on the top shelf of the closet.
She was seated on the edge of her mattress pad, and her dangling sandaled feet nearly touched the spotless white floor.
She crept in, found a small crevice behind a pile of boxes, and pointed it out to the drowsy child, who crawled in with his little mattress pad while she piled the packaged food and drink behind him.