The term also has closely related synonyms that are employed throughout the Quran. Each synonym possesses its own distinct meaning, but its use may converge with that of qurʼān in certain contexts. Such terms include kitāb (book); āyah (sign); and sūrah (scripture). The latter two terms also denote units of revelation. In the large majority of contexts, usually with a definite article (al-), the word is referred to as the "revelation" (waḥy), that which has been "sent down" (tanzīl) at intervals. Other related words are: dhikr (remembrance), used to refer to the Quran in the sense of a reminder and warning, and ḥikmah (wisdom), sometimes referring to the revelation or part of it.