契 is a classical Chinese character with multiple meanings. In modern Mandarin it is a bound morpheme used primarily in compounds. Its most common sense is as a noun meaning 'contract' or 'deed', appearing in words like 契约 (contract) and 地契 (land deed). It also functions as a verb meaning 'to agree' or 'to be in accord', as in 默契 (tacit understanding) and 契合 (to coincide). An older meaning, 'to carve', survives in literary contexts like 契刻 (to carve). You will encounter 契 most often in formal, legal, or literary writing.