double blind

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

• noun A testing procedure designed to avoid biased results by ensuring that at the time of the test neither the administrators nor the subjects know which subjects are receiving a test treatment and which belong to a control group.

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

• noun a test procedure in which the identity of those receiving the intervention is concealed from both the administrators and the subjects until after the test is completed; designed to reduce or eliminate bias in the results