The Oral Torah תורה שבעל פה
The teaching received together with the written Torah and transmitted by word of mouth until it was written down.
The written Torah almost nowhere explains how to do what it commands. It says «do no work on the Sabbath» — but what work; «slaughter as I have commanded you» — though nowhere is the manner given; «a sign upon your hand» — but what sign. The answers lie in the Oral Torah, received, by tradition, at Sinai together with the written text.
The chain is set out at the start of Pirkei Avot: «Moshe received the Torah at Sinai and handed it to Yehoshua, Yehoshua to the elders, the elders to the prophets, the prophets to the Men of the Great Assembly».
Writing it down began under duress — the Mishna around 3948, the Talmud by 4260 — under the rule «it is a time to act for the Lord». Denying the Oral Torah defined the Sadducees and later the Karaites.