1. Biology An immature, embryonic stage in the development of cells or tissues: erythroblast.
2. Geology A crystal or mineral deposit formed in metamorphic rock: porphyroblast.
[From Greek blastos, bud, germ, of unknown origin.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.