宪 is a bound morpheme (b.f.) meaning 'constitution' or 'statute' — it never stands alone and always appears as part of a compound word or phrase. You'll most often encounter it in 宪法 (xiànfǎ, 'constitution'), 违宪 (wéixiàn, 'unconstitutional'), and 宪政 (xiànzhèng, 'constitutional government'). Because it's a literary/formal morpheme, it's almost exclusively used in legal or political contexts.