C clef

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

• noun A symbol indicating which line of a staff represents the pitch of middle C. On the bottom line it becomes the soprano clef, on the middle line the alto clef, and on the third line above the bottom the tenor clef.

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

• noun music A clef indicating the position of one-lined C on a staff.

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

• noun a movable clef that puts middle C on one of the lines of a staff