ekvilibro

The trap first: this is not libro. Everything in a learner trained on Esperanto affixes says to cut ekvilibro into something plus libro, a book. There is no reading of the word in which that helps. Ekvi- is not a prefix in Esperanto, and even in Latin the pieces are aequus and libra, not liber. Libro the book comes from Latin liber, the inner bark people wrote on before paper; libra is a set of weighing scales. Two different words that happen to look alike, and the resemblance is an accident.

Now the gift. Once you know it is aequus plus libra the word paints a picture: EQUAL SCALES, the beam dead level, neither pan tipping. That is what balance is, and it is where the star sign Libra comes from, and where the pound sign gets its L, and - by way of Latin libella, a little balance - where a spirit level gets its name. Esperanto borrowed the finished Latin compound, so the seam is real history and not a rule you can reuse: it belongs with inspiri and privilegio in the class of words that decompose beautifully in Latin and not at all here.

How it is used. Physical balance - perdi la ekvilibron, to lose your balance, konservi la ekvilibron, to keep it - and every figurative extension English makes: the balance of power, a balanced diet, mental equilibrium. The verb ekvilibri is to balance something, ekvilibra is balanced, and senekvilibra unbalanced. Ekvilibristo is the tightrope walker, which is one of the more satisfying -ist- words in the language.