half hitch

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

• noun A knot or hitch made by looping a rope or strap around an object and then back around itself, bringing the end of the rope through the loop.

from The Century Dictionary.

• noun A hitch formed by passing the end of a rope round its standing part and bringing it up through the bight. See hitch.
• noun In pillow-lace making, the loop by means of which the thread is tightened upon the bobbin. Also called rolling-hitch.

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

• noun A knot or hitch made by passing the end of the rope once around the standing part or other rope.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a knot used to fasten a rope temporarily to an object; usually tied double