And probably for whoever composed this "definition." The Tosefta is not part of the Talmud. The Talmud is Mishna and Gemara, the latter being an explication of the former.
The Tosefta is a work similar to the Mishna in that it is a compilation of laws that lend detail to the laws in the Torah. Effectively it lost out to the Mishna in "official" status. It is not clear whether it came before, after, or overlapped the Mishna as a written document.
ML/NJ
Thanks for the information — do you have a web source?