Kabbalah is understood by those who study it to be the collection of mystical knowledge within the milieu of the Hebrew faith.
The word kabbalah comes from the Hebrew קבל (qabal), to receive; literally translated, kabbalah means that which is received, alluding to its divine origin. Kabbalistic knowledge was traditionally guarded from publication at large, being reserved for only the select few who merited its reception. It was typically transmitted orally, being occasionally documented in private collections, many of which are irretrievably lost to the ravages of time.
Historically, the word kabbalah itself was applied to all non-published Judaic knowledge, including the Mishnah, Talmud, and other orally transmitted information. What we understand to be kabbalah today was originally known by many other names, including Torah HaSod (the secret Torah).