What does "blind" mean in the context of a clinical trial?

A blind or blinded-experiment is an experiment in which some information about the test is masked from the participants. These do not know if they are using the drug or a placebo. This methodology is employed in order to avoid any kind of bias until the trial has ended.

There are several anonymization methodologies for the information associated to a clinical trial:

• When the patient is not aware of the treatment he/she is receiving: single-blind.
• When both the patient and the doctor/tester do not know the treatment: double-blind.