tranokti

Decode it and you have the whole word. Tra is through, nokto is night, and -i makes a verb. To night-through. English needs four words for it - to spend the night - and Esperanto needs one, which is the kind of small win that makes people evangelical about the language. German does the same trick with übernachten, and the parallel is not a coincidence: Zamenhof was building the pattern he could see working in the languages around him.

Note what the preposition is doing. Tra here is not spatial. You are not passing through the night the way a train passes through a tunnel; you are occupying it end to end, which is exactly the temporal sense tra carries in tra la tuta tago, all day long. Compounding is full of prepositions used this way, and reading them as duration rather than movement unlocks a lot of vocabulary at once.

How it behaves. It is intransitive - you do not tranokti a place, you tranokti in one - and it takes the ordinary prepositions: tranokti en hotelo, tranokti ĉe amikoj, tranokti sub la ĉielo. Tranokto is the overnight stay as a noun, and tranoktejo is the place you do it, which is what the small print of a hostel booking will call itself. The pattern is productive too: tramateni and travintri are both built the same way, and a reader meets them without needing them explained.