"According to
Muhaddithiin [scholars of hadith] it stands for 'what was
transmitted on the authority of the Prophet, his deeds, sayings,
tacit approval, or description of his sifaat (features) meaning his
physical appearance. However, physical appearance of the Prophet is
not included in the definition used by the jurists.' "
"Thus hadith
literature means the literature which consists of the narrations of
the life of the Prophet and the things approved by him. However, the
term was used sometimes in much broader sense to cover the
narrations about the Companions [of the Prophet] and Successors [to
the Companions] as well."
M. M. Azami, "Studies
in Hadith Methodology and Literature."
Quoted in "Sunnah and Hadith," at:
http://www.usc.edu/dept/MSA/