1. Biology. Similarity of external form or appearance but not of structure or origin.
2. Zoology. A resemblance in form between the immature and adult stages of an animal.
3. Mathematics. A transformation of one set into another that preserves in the second set the operations between the members of the first set.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.