Any of a number of different protein factors which, when acting together, can form a blood clot shortly after platelets have broken at the site of the wound.
The factors have Roman numeral names, like VII, VIII, IX, X, XI, and XIII. Defects in the genes which code for any of these factors result in genetic diseases like hemophilia, which results from a defect in the gene for factor VIII or IX.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.