What is "tosafot"

The Tosafot or Tosafos are medieval commentaries on the Talmud. They take the form of critical and explanatory glosses, printed, in almost all Talmud editions, on the outer margin and opposite Rashi's notes.

The authors of the Tosafot are known as Tosafists ("ba'ale ha-tosafot"); for a listing see List of Tosafists.