因 (yīn) is a formal, written way to say 'because of / due to' — it is shorter and more literary than the everyday 因为 (yīn wèi). It often appears in fixed expressions (因此, 因而) and is common in formal writing. 由于 (yóu yú) is similarly formal but typically introduces a full reason clause, while 因 can also be used before a noun phrase.