Definition
It is a kind of study/experiment where the both the examiner and the test candidate are unaware of the result of the study.
A horse named Hans
In the 1900 there was a horse that allegedly had the intelligence of a teenager. It would answer your questions by stomping it’s hooves. Turns out that horses are extremely capable of detecting slight shifts in facial muscles. So when the examiner didn’t know the answer to the question, the horse just didn’t know when to stop stomping.
This led to the creation of the double blind test. Which means that neither the examiner nor the test taker knows what is what. Extremely useful in testing out placebos.