rhom·boid (r?m?boid?)n. A parallelogram with unequal adjacent sides, especially one having oblique angles.adj. also rhom·boi·dal (-boid?l) Shaped like a rhombus or rhomboid.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.