A sheet (usually with elastic edges) tailored to fit a particular mattress WordNet 3.0 & Open English WordNet

A sheet tailored to form pocket for a mattress with elastic edges that is suitable for a range of mattress sizes. The edge may also contain a drawstring to secure the sheet more firmly and enable use on a wider range of mattress thicknesses. Wiktionary

Sheet with elastic edges. Wiktionary (translation)