kateno

A kateno binds a prisoner. A ĉeno holds a bicycle together. Both are chains of a sort, and Esperanto splits them so the metaphor cannot get lost: ĉeno is the neutral mechanical object, kateno is always about captivity, whether a real iron one on a real ankle or the figurative kind in a political speech. Rompi la katenojn is a movement phrase you will meet before long.

The plural is the normal form. Fetters come in pairs and Esperanto follows the same instinct as English, so katenoj is what people write. Kateni is the verb, to shackle or to fetter, and enkatenigi puts someone into them.

There is no -en- suffix in here. Learners who have met the direction ending on tien and hejmen sometimes read the -en- in kateno as something separable, but the accusative of direction attaches to whole words and produces adverbs, not nouns. Katen- is one root, and what is left after you take the -en- away is kat, a cat, which is a coincidence and a good way to remember that the seam is imaginary.