fakulo

Two pieces, no surprises. Fako is a compartment, and by an easy extension a field of study or a department of a business - the pigeonhole sense and the subject sense sit in the one root, exactly as German Fach carries both. Add -ul-, the suffix for a person characterised by the root, and a fakulo is a person of a particular field: the one who knows this compartment and not the others.

The -ul- family is worth learning as a set. Riĉulo a rich man, junulo a youth, sanktulo a saint, malsanulo a patient, kuraĝulo a brave one, stultulo a fool. The suffix does not mean "man" - it means "person marked by this", and it takes -in- for a woman when the sex matters (fakulino). The affixes for size and people covers the whole group, and once you have it you stop needing separate vocabulary for the "-ist, -er, -eer" tangle English makes of the same job.

Specialist rather than master. A fakulo is defined by the narrowness of the field, not by how good they are - that is the honest translation. Majstro is a master, a matter of attainment; spertulo is an experienced person; fakulo is the one whose fako this is, and the word carries a faint dryness about it that English "expert" also has when politicians use it. The related build fakisto (with -ist-, the professional suffix) exists but is rarer; and faka as an adjective means specialised or technical, so faka vortaro is a technical dictionary and fakliteraturo the specialist literature.