"art of curing mental diseases," 1892, from psycho- + therapy, on model of French psychothérapie (1889). In early use also of treatment of diseases by "psychic" methods (mainly hypnotism). Psychotherapeia was used in medical writing in 1853 as "remedial influence of the mind." Related: Psychotherapeutic (1890, in reference to hypnotic treatment); psychotherapeutics (1872).
word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.
Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.
Others are reading
Definitions of psychotherapist from WordNet
a therapist who deals with mental and emotional disorders;