psikologio

A family, not a word. -Ologio is the international ending for a field of study, and Esperanto carries it as freely as English does. You already have filozofio at rank 1,645 and biologio at 2,077, and around them sit geografio, arkeologio, sociologio, teologio, etnologio and the rest. Meet a new one and you can price it up: the first half names the subject, the -logio says somebody studies it systematically.

The paradigm is completely regular. Psikologio is the field. Psikologo is the person - not psikologiisto, because the -log- ending already carries the practitioner in the international form the language borrowed. Psikologia is the adjective, psikologie the adverb. That last one is worth practising: psikologie parolante, psychologically speaking, is a phrase Esperanto builds in one word where English needs two.

The root underneath. Psiko is a word in its own right - the psyche, the mind considered as a whole - and psika means psychic or mental. Beside it sit menso, the mind as the thing that thinks, and cerbo, the physical brain. Esperanto keeps all three apart, which is more precision than English manages with mind.

Why this matters beyond the one word. Scientific and technical vocabulary is where the language's borrowing policy is most visible: the roots come in whole from the international stock, the endings come from Esperanto, and the result is that a chemist or a psychologist can read a paper in the language after a fortnight. That trade, and where it strains, is the subject of scientific Esperanto.