Source : Webster's Revised Unabridged Dictionary (1913)

Pillowcase \Pil"low*case`\, n.
   A removable case or covering for a pillow, usually of white
   linen or cotton cloth.

Source : WordNet®

pillowcase
     n : bed linen consisting of a cover for a pillow; "the burglar
         carried his loot in a pillowcase" [syn: {case}, {slip}, {pillow
         slip}]