The Collaborative International Dictionary of English v.0.48:

Pillowcase \Pil"low*case`\, n.
   A removable case or covering for a pillow, usually of white
   linen or cotton cloth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pillowcase
    n 1: bed linen consisting of a cover for a pillow; "the burglar
         carried his loot in a pillowcase" [syn: case,
         pillowcase, slip, pillow slip]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "pillowcase":
   afghan, bed linen, bedclothes, bedcover, bedding, bedsheet,
   bedspread, blanket, buffalo robe, case, clothes, comfort,
   comforter, contour sheet, counterpane, cover, coverlet, coverlid,
   eiderdown, fitted sheet, lap robe, linen, patchwork quilt,
   pillow slip, quilt, robe, rug, sheet, sheeting, slip, spread