المعنى الاصطلاحي :

Emphasizing or confirming a matter by swearing by a sacred being preceded by one of the following three Arabic letters: "bā’", "waw", "tā’" (as in saying: "billāhi", "wallāhi", "tallāhi"; all mean: by Allah).

الشرح المختصر :

"Qasam" (oath) is a way of confirming something. It consists of the oath wording, the thing by which one swears, and the thing for which one swears. The oath wording could be the Arabic letters "waw", "tā’", or "bā’" (such as saying: "billāhi"), or verbs (such as saying: "ahlifu", "uqsimu"), or nouns (such as saying: "yameen-ullāhi"). Taking an oath is permissible if one swears by Allah, the Almighty; His beautiful Names; or His sublime Attributes. As for swearing by other than Allah, it is considered major "shirk" (associating partners with Allah) if the swearer believes that the one he is swearing by is equal to Allah, the Almighty, in glory and majesty; otherwise, it is considered minor "shirk".

التعريف اللغوي المختصر :

"Qasam": swearing, taking an oath. Derived from "qasāmah", which means the oaths that are divided and distributed among the blood relatives of the slain person (compurgation by oath; the practice of clearing a person accused of murder by the oaths of others testifying to his innocence).