1650s, "the study of the soul," from Modern Latin psychologia, probably coined mid-16c. in Germany by Melanchthon from Latinized form of Greek psykhē "breath, spirit, soul" (see psyche) + logia "study of" (see -logy). The meaning "science or study of the phenomena of the mind" is attested by 1748, in reference to Christian Wolff's "Psychologia empirica" (1732). The modern behavioral sciences sense is from the early 1890s.
short for psychology in various senses; e.g. as an academic study, in student slang by 1895.
Similarly, [Prof. Hans Driesch] includes under "parapsychology" such phenomena as telepathy and clairvoyance, which he regards as mere extensions from ordinary mental phenomena, rather than as fundamentally different processes. He believes that the same orderly process by which unclassified and diverse processes have been systematized,—alchemy becoming chemistry, astrology becoming astronomy,—is at work now,—to make, in place of the mysterious tradition of Occultism, a science which will really be an extension from scientific psychology and biology. ["Journal of the American Society for Psychical Research," April 1923]
1680s, "of or pertaining to the mind as a subject of study;" see psychology + -ical. In early 20c. the sense gradually shifted toward "affecting or pertaining to a person's mental or emotional state." Related: Psychologically. Psychological warfare "use of propaganda, etc., to undermine an enemy's morale or resolve" is recorded from 1940. Psychological moment was in vogue from 1871, from French moment psychologique "moment of immediate expectation of something about to happen."
The original German phrase, misinterpreted by the French & imported together with its false sense into English, meant the psychic factor, the mental effect, the influence exerted by a state of mind, & not a point of time at all, das Moment in German corresponding to our momentum, not our moment. [Fowler]