psiko

The base has arrived, four hundred ranks late.Psikologio landed at 2,371 and had to be taught as an unopenable borrowing, because the root it is built on was not yet in the course. Here it is. Psiko plus -logi- (the study of) gives psikologio; psika is the adjective, mental or psychical; psikologo is the practitioner and psikologia describes the discipline. From the same root you get psikiatrio (the branch of medicine), psikanalizo (Freud's method) and psikozo. That is a large shelf of vocabulary opened by one rank-2,763 word, which is the usual bargain in Esperanto's international layer.

Where it sits among the four mind words. This is the distinction learners ask about, and Esperanto keeps all four apart. Cerbo at 829 is the organ, the wet grey thing, and a surgeon operates on it. Menso at 1,073 is the thinking apparatus, the mind that solves a problem or forgets a name. Animo at 233 is the soul, the part that religion argues about and poems address. Spirito at 404 is spirit in the wide sense, from a mood to a ghost to the spirit of an age. And psiko is the technical one: the whole mental life of a person, conscious and unconscious together, as a discipline studies it. PIV defines it as the seat of all conscious and unconscious inclinations, understandings, emotions and volitions - which is a definition, not a synonym, and the word carries that clinical weight.

A spelling note worth having. Older texts write psiĥo, with the ĥ, and some dictionaries still list both. The ĥ has been retreating across the language for a century, covered in the ĥ question, and psiko with a plain k is what modern writing uses. Both are correct; only one will look normal to a reader in 2026.