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Any of various organisms that require extreme conditions of temperature, pressure, or chemical concentration, as in very cold or salty environments, in order to thrive.
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.
(Microbiology) a microbe that lives in an environment once thought to be uninhabitable, for example in boiling or frozen water
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014