prizono

A loan and a build, side by side. Esperanto has two words for this and you will meet both. Malliberejo is the Zamenhofan one, assembled out of parts you already own: mal- (opposite) plus libera (free) plus -ej- (place), so literally an un-free place. Prizono is the international loan. They mean the same thing.

Which does usage prefer? Honestly, the built one. Malliberejo is commoner in Esperanto prose, and it is what the older literature almost always uses. It is longer, it is unmistakable, and it is the sort of word that made people fall in love with the language in the first place, because a learner who knows three morphemes can read it cold. Prizono turns up in translated fiction and in journalism, where malliberejo can feel ponderous in a headline, and it is fully standard: PIV lists it without complaint. Write either; recognise both.

The people and the parts. Malliberulo or prizonulo is a prisoner, prizonestro the governor, ĉelo the cell, gardisto the warder from gardi, to guard. Enprizonigi is to imprison, and liberigi is to release. Kondamni is to sentence someone, and what they lose is libereco.

Do not confuse it with karcero. Karcero is the lock-up: a cell for short detention, a guardhouse, the punishment cell inside a prison. A prizono is the whole institution.