Baraita: what was left outside the Mishnah ברייתא
Rabbi Yehuda HaNasi did not take everything into the Mishnah. What was left outside was not lost: it was taught and handed on, and the Gemara says again and again "tanu rabanan"—"the sages taught"—and brings it.
The word is Aramaic: baraita, "the outside one." So is called the teaching of the Tannaim that did not enter the Mishnah.
Where it comes from. The Mishnah is a selection. Rabbi Yehuda HaNasi, compiling it, chose: what to keep, in which wording, whose opinion to place first. The rest went on being learned by heart in the houses of study.
Where it survived. In the Tosefta—a collection following the order of the Mishnah and supplementing it. In the halakhic midrashim: Mechilta, Sifra, Sifrei. And inside the Gemara itself, where a baraita is introduced by "tanu rabanan," "tanya" or "tanei."
What the Gemara needs it for. A baraita is an argument. It is of the same generation as the Mishnah, and so one may object with it: if the baraita says otherwise than the Mishnah is understood to say, then it is understood wrongly. Half of Talmudic analysis is built exactly so.
Not every baraita is equal. The sages distinguished: a baraita that came through a reliable transmitter was accepted, a doubtful one set aside—"this baraita was not taught in the school of Rabbi Chiya and Rabbi Oshaya," that is, it did not pass a tested school.
On the timeline
Related
Sources
Chazal: Mishnah, Talmud, midrash, Zohar
- Tosefta — the chief collection of surviving baraitot, following the order of the Mishnah
- Chullin 141a — “this baraita was not taught in the school of Rabbi Chiya and Rabbi Oshaya” — how a reliable baraita was told from a doubtful one
Basis: Talmud and midrash.