The horizontal member that bears the upright portion of a frame, especially the horizontal member that forms the base of a window.
Geology An approximately horizontal sheet of igneous rock intruded between older rock beds.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.