婚 (hūn) is a bound morpheme (b.f.) — it cannot stand alone as an independent word. It appears only in compound words related to marriage, such as 结婚 (jiéhūn, to get married), 婚礼 (hūnlǐ, wedding), 婚姻 (hūnyīn, marriage), and 离婚 (líhūn, to divorce). Think of it as the core 'marriage' syllable attached to other characters.