organo

Three senses, and the dictionary is stricter about them than English is. First and commonest, the body part: la koro estas organo, and the whole of anatomy follows. Second, an institution's mouthpiece, the journal or paper that speaks for a body: la oficiala organo de la asocio is standard Esperanto and turns up constantly in movement history, where every society had one. Third, and drier, an organ in the sense of an agency or a means by which something acts, as in la organoj de la ŝtato.

Now the correction, because it is the one that costs marks. The musical instrument is not organo. It is orgeno. The church organ, the pipes, the thing with the pedals: orgeno, and an organist is an orgenisto. English collapsed the two words centuries ago and everybody has since had to live with the joke; Esperanto kept them apart, which is the better arrangement and the sort of thing the language does well. If you want an instrument in the general sense, instrumento is the word, and it covers the scientific and the musical alike.

The derivations are worth a line. Organismo is an organism, organizi is to organise, and organiza is organisational. All three grew from the same Greek ergon, work, which is also sitting inside energio. That is a lot of vocabulary hanging off one small root, and noticing it is cheaper than learning the words one at a time.