In religious terminology, the term hadith is used for "the words of the Prophet", and later came to be used for "his words, deeds and explanations". Moreover, the words and deeds of the Companions and the following generation -with even a reference to maqtu (sayings of the following generation) and mauquf (sayings of the Companions) - are also called hadith. In this sense, there are also scholars who use the word khabar (report) instead of hadith, and the term asar for the words and deeds of Companions and the following generation.

In much broader sense, hadith can be defined as follows:

"Everything attributed to Prophet Muhammad (or the Companions or following generation), such as words, deeds, explanations, or characteristics of his creation and character."

The word Sunnah literally means "path and behavior". Although to start with the term sunnah was used for "the deeds of the Prophet", and hadith for "the words of the Prophet", Sunnah has become a term that is used for all direct explanations by the Prophet through words or deeds and his indirect explanations by tacit approval of events he heard or witnessed. According to scholars of usul al-fiqh, this is the definition of Sunnah. However, hadith scholars use the words hadith and Sunnah interchangeably.