occidens

Occidens, occidentis, m., third declension. It is not really a noun but the present participle of occido, "fall, set", left standing on its own with sol understood: the setting sun, therefore the west. Oriens is built the same way from orior, and the two are always learnt as a pair.

The trap is the homograph. Occido, occidere, occidi, occasum, "fall, set", has a short i and is the one you want here; occido, occidere, occidi, occisum, "kill", has a long one, from ob plus caedo. Same spelling on the page, different supine, wholly different sense. In the medieval corpus occidens is usually the Latin West set against the Greek East.