Not changing or subject to change; constant.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "invariable" in a sentence
"It is this that constitutes the invariable laws of motion: I say _invariable_, because they can never change, without producing confusion in the essence of things."
"The article is invariable, that is, does not change in form when used with plural nouns, as "la viro", the man, "la viroj", the men."
"Dan Treu grinned as he recalled the invariable exchange of personalities when they met."