The Mishnah (Hebrew משנה, "repetition"), redacted circa 200 CE by Yehudah Ha-Nasi (יהודה הנשׂיא / "Judah the Prince"), is the first written recording of the Oral Torah of the Jewish people, as championed by the Pharisees, and as debated between 70-200 CE by the group of rabbinic sages known as the Tannaim. It is considered the first work of Rabbinic Judaism and is a major source of Rabbinic Judaism's religious texts: Rabbinic commentaries on the Mishnah over the three centuries after its composition were then redacted as the Gemara (Aramaic: "Tradition").
In traditional Jewish belief, the "Oral Torah", or "oral law," was an unwritten tradition which was given from God to Moses on Mount Sinai, which elucidated the written Torah but was not incorporated into its text. However, the Talmud relates that the oral laws were eventually recorded in the form of the Mishnah by Rabbi Yehudah HaNasi, referred to in its text simply as "rabbi," when the passage of time and the persecution of the Jews raised the possibility that its details would be forgotten.
The Mishnah consists of six orders ("sedarim"), each containing 7-12 tractates called masechtot, each of which is divided into verses called mishnayot. The orders include: Zeraim ("Seeds"), dealing with agricultural laws and prayers, Moed ("Festival"), pertaining to the laws of the Sabbath and the Festivals, Nashim ("Women"), concerning marriage and divorce, Nezikin ("Damages"), dealing with civil and criminal law, Kodashim ("Holy things"), regarding sacrificial rites, the Temple, and the dietary laws, and Tohorot ("Purities"), pertaining to the laws of purity and impurity, including the impurity of the dead, the laws of ritual purity for the priests (Kohanim), the laws of "family purity" (the menstrual laws) and others.
The word mishna (plural: mishnayot) can also indicate a single paragraph of the work itself, i.e., the smallest unit of structure in the Mishnah. Thus, a number of mishnayot make up a perek (chapter), a number of perakim (chapters) make up a masechet (tractate), a number of masechtot (tractates) make up a seder (order) and the term Shas (an acronym for Shisha Sedarim - the "six orders") may refer to the complete Mishnah. Common modern editions of the Talmud have each mishnah followed by its associated Gemara commentary. Then, the next Mishnah, often only a few lines or short paragraph, followed by the commentary relevant to that Mishnah which may be pages long, and so on, until that particular tractate of Mishnah is completed. There may be many chapters of Mishnah (Ma'sechta) in any given tractate.