The word thus simultaneously refers to a science (involving the study of the behavior of humans and animals) and to various interventions (treatments and therapies) in the mental processes and behavioral patterns of people. It is a branch of science that studies the cognitive, perceptual, and behavioural characteristics of human beings. The "how and why" of our behavior.
Jill: It's so stressful, I have started throwing up all my food again, and I hate my family, and I hear voices, and I think I might have the ugliest nose on the continental United States.
Jack: What's your major?
Jill: Psychology! I'm going to help people someday!