retardant

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Acting or tending to retard. Often used in combination.
• noun A substance that retards a process, as a flame retardant.

from The Century Dictionary.

• Retarding; tending to delay or impede motion, growth, or progress.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective often in combination Serving to retard (slow down) the action of something
• noun often in combination Something that serves to retard (slow down) the action of something

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any agent that retards or delays or hinders