from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Of or relating to a heating or cooling system that transfers heat by circulating a fluid through a closed system of pipes.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Refers to heating systems which involve circulating hot water or steam.
hydr(o)- + -onic (as in electronic).(American Heritage® Dictionary of the English Language, Fourth Edition)
blend of hydro- and -onic (Wiktionary)