Blinding is a way of making sure that the people involved in a research study, such as the participants in clinical trials, do not know which trial arm they are assigned to. For example, in a trial with one treatment arm and one placeboarm, blinding means that the participants do not know if they are receiving the treatment or the placebo.
Blinding is used to remove bias that can be caused intentionally or unintentionally if participants or the research team are aware of which trial group participants are in.
Sometimes the term 'single-blind' is used to describe studies where the participants are unaware of which arm they are in, but the research team does know. In a double-blind trial, both the research team and participants do not know which participant is assigned to which arm.
A blind trial is the opposite of an open or open-label trial.