kalifo

One root, no seams. Kalifo looks as though it might break into pieces, and it does not. There is no kal- and no -ifo; the whole word is a single borrowing from Arabic, and the only thing you add to it is the ordinary -o that makes it a noun. The derived words are the predictable ones: kalifujo or kalifolando for the caliphate as a territory, kalifeco for the office itself.

What the word actually means, because the translation hides it. English caliph is opaque; Arabic khalīfa is not. It means successor or deputy, from a verb meaning to come along behind. That is the whole constitutional claim in one word: the caliph succeeds la profeton as leader of the community and is emphatically not a prophet in his own right. When you meet the word in an Esperanto history text, read it as "the successor" and the sentence usually explains itself.

Where it sits in the vocabulary you already have. A reĝo rules a reĝlandon and inherits by blood; a caliph heads both a state and a religion at once, which is why the word survives in historio rather than in the language of modern government. Beside it sit sultano, emiro and imamo, none of which are interchangeable, and the buildings of the same world: moskeo at rank 2,506, which arrived in this course four hundred ranks ago.