the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
A mutawâtir hadîth is a hadîth that was independently related by such a large number of people that is practically impossible for them to have been able to conspire on a lie or collectively be mistaken. This large number of narrators must exist at every point in the chain of transmission until it reaches its final source.
This type of hadîth is guaranteed to be authentic, since it is impossible for it to be a fabrication or for it to contain within it a mistake. The number of narrators required for a hadîth to be mutawâtir varies depending on the circumstances. The important thing is that it would have been impossible for such a large number of people, given their circumstances, to have conspired in fabricating the report.
Any hadîth that is not mutawâtir is called a single narrator hadîth (khabar âhâd). This does not mean that there is only one narrator. It only means that the number of narrators who independently related the hadîth is less than the number required for the hadîth to be classified as mutawâtir.
This type of hadîth is not guaranteed to be authentic. In order to determine the authenticity of the hadîth, its chain of transmission must be heavily scrutinized. The hadîth can then be graded as authentic (sahîh), good (hasan), weak (da`îf), rejected (munkar), or fabricated (mawdû`).
Source: Islam Today