hospital corner

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tight-fitting triangular fold made by tucking a sheet and blanket securely under a mattress on the end and on each side at the corners.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A tight and precise method of tucking a sheet or blanket under the edges of a mattress, so as to form crisp folds at the corners of the bed.