A protective pad used to cover a mattress and guard it against soiling is called a mattress pad.
A mattress pad usually lies on top of the mattress and beneath the fitted sheet. It may be quilted or padded, adding softness and helping shield the mattress from sweat, skin oils, dust and minor spills. Many versions are washable, making them easier to clean than the mattress itself.
The term is often confused with “mattress protector.” A protector is primarily designed as a barrier against stains, moisture, allergens or pests, and may be waterproof or fully encased. A mattress pad can provide some protection, but its main purpose is often comfort and cushioning. In everyday retail language, however, the two terms can overlap.
For heavy liquid protection, a waterproof mattress protector is generally the more precise choice. For a padded, washable layer that protects against ordinary soiling, “mattress pad” is the standard answer.