slip ring

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

• noun A metal ring mounted on a rotating part of a machine to provide a continuous electrical connection through brushes on stationary contacts.

from The Century Dictionary.

• noun A metal ring, usually of copper or cast-iron, used on electric apparatus to lead the current into a revolving structure.

from the GNU version of the Collaborative International Dictionary of English.

• noun a metal ring on a rotating part of a machine to provide a continuous electrical connection through brushes on stationary contacts.

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

• noun A component of an electromechanical device which, in combination with brushes, provides a continuous electrical connection between stationary and rotating conductors

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

• noun connection consisting of a metal ring on a rotating part of a machine; provides a continuous electrical connection through brushes on stationary contacts