Set and setting

Set and setting, when referring to a psychedelic drug experience, means one's mindset (shortened to "set") and the physical and social environment (the setting) in which the user has the experience. This is especially relevant for psychedelic experiences in either a therapeutic or recreational context. According to the book "How to Change Your Mind" by Michael Pollan, the concept of set and setting was observed by the "Johnny Appleseed" of LSD, Al Hubbard visiting mushroom ceremonies in Mexico. The terms were used at least as early as 1958 by Ludwig von Bertalanffy and popularized by Timothy Leary in 1961, and became widely accepted by researchers in psychedelic therapy. Norman Zinberg has also discussed this in "Drug, Set, And Setting: The Basis for Controlled Intoxicant Use" (1984).
"Set" is the mental state a person brings to the experience, like thoughts, mood and expectations. "Setting" is the physical and social environment. Social support networks have shown to be particularly important in the outcome of the psychedelic experience. They are able to control or guide the course of the experience, both consciously and subconsciously. Stress, fear, or a disagreeable environment, may result in an unpleasant experience (bad trip). Conversely, a relaxed, curious person in a warm, comfortable and safe place is more likely to have a pleasant experience. A curated playlist can be part of a favourable setting. 
Due to the importance of setting in early psychedelic therapy, Hubbard introduced a "treatment space decorated to feel more like a home than a hospital", which came to be known as a "Hubbard Room".
In 1966, Timothy Leary conducted a series of experiments with dimethyltryptamine (DMT) with controlled set and setting. The aim was to see whether DMT, which had then been mostly thought of as a terror-inducing drug, could produce pleasant experiences under a supportive set and setting. It was found that it could.
Set and setting has also been investigated from a religious perspective.