Mishnah משנה
The first writing-down of the Oral Torah: six orders, redacted by Rabbi Yehuda haNasi around 3948.
Before the Mishna the Oral Torah was deliberately not written down: it passed mouth to mouth, and that was its nature. But after the Temple's destruction, the Bar Kochba revolt and Hadrian's persecutions it became clear the chain could break. Rabbi Yehuda haNasi, who combined learning with authority, gathered what had accumulated and gave it form.
The Mishna has six orders: Zeraim (agriculture and blessings), Moed (festivals), Nashim (marriage and divorce), Nezikin (damages and courts, including Pirkei Avot), Kodashim (Temple and offerings), Taharot (purity) — sixty-three tractates in all.
Its language is extremely terse; it records disputes more often than conclusions and preserves rejected opinions so that they are known to have been weighed. Both Talmuds are built on it, line by line.