keel 1 (k?l)n.1. Nautical a. The principal structural member of a boat or ship, running along the center of the hull from bow to stern, to which the ribs are attached.b. A projecting ridge or fin on the bottom of the hull of a boat or ship that improves directional control and is often weighted for added stability.2. The principal structural member of an aircraft, resembling a ship’s keel in shape and function.3. A structure, such as the breastbone of a bird, that resembles a ship’s keel in function or shape.4. A pair of united petals in certain flowers, as those of many members of the pea family.intr. & tr.v. keeled, keel·ing, keels Nautical To capsize or cause to capsize.Phrasal Verb: keel over To collapse or fall into or as if into a faint.[Middle English kele, from Old Norse kjölr.]keel 2 (k?l)n.1. Nautical a. A sail-powered barge, especially one historically used on the rivers of northern England.b. The load capacity of this barge.2. A British unit of weight formerly used for coal, equal to about 21.2 long tons.[Middle English kele, from Middle Dutch kiel.]keel 3 (k?l)tr.v. keeled, keel·ing, keels Chiefly British To make cool.[Middle English kelen, from Old English c?lan, to cool; see gel- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.