Dictionary.com
Thesaurus.com

HII region

American  

noun

plural

HII regions

  1. an area of interstellar space containing clouds of ionized hydrogen.

HII region Scientific  
/ āch′to̅o̅′ /

  1. A cloudlike region in space where the interstellar medium consists of ionized rather than neutral hydrogen. HII regions are centered around stars with very high surface temperatures and brilliant luminosities, whose radiant energy is sufficient to heat the interstellar medium to the point that hydrogen is ionized. The most conspicuous and best-known HII region is the Orion Nebula.

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.