reaction time

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

• noun The interval of time between application of a stimulus and detection of a response.

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

• noun The interval between the reception of a stimulus and the initiation of a response

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

• noun the time that elapses between a stimulus and the response to it