kodo

Three senses, in the order the dictionary gives them. A kodo is first a system of signs or signals for carrying messages: la Morsa kodo, Morse code, is the textbook case and a genuinely good one, because it makes the definition visible. Dots and dashes mean nothing on their own and everything by agreement. Second, a kodo is a particular group of signs that identifies something, which is the sense in a barcode, a postcode or a pasvorto, a password. Third, and only recently, it is program code: fontkodo is source code.

The word that is not this word. A code of laws is a kodekso, not a kodo. La civila kodekso is the civil code; la Kodekso de Hamurabo is the code of Hammurabi. Both words come from the same Latin codex, but Esperanto took it twice and split the jobs, keeping the fuller form for the law book and the shorter one for the system of signs. English uses one word for both and has to lean on context; here you choose, and choosing wrongly is noticeable.

The verbs are all regular. Kodi is to encode, kodita mesaĝo an encoded message, malkodi to decode with the ordinary opposite prefix, kodado the process of encoding, kodaĵo the encoded thing itself. Kodigi is to make somebody or something encode, which is a different verb from kodi and worth noticing, because the -ig- is doing its causative job rather than padding.

A note on typing. The reason a page like this one needs to say sxtipo as well as ŝtipo is the same reason codes exist at all: keyboards and channels have limits, so people agree on substitutions. The x-system is a code in the strict sense of this word's first meaning, and it is fully reversible, which is what separates a code from a mangling.