Factor  refers to several substances or activities that are necessary to produce a result. In the context of blood thinners, coagulations, and TIA's,  it refers to the type of protein that helps regulate the time it takes to make coagulation (stopping bleeding).

Factor VIII (factor eight), is a key factor in the process of blood coagulation (clotting). Lacking normal factor VIII causes hemophilia. It is also known as antihemophiliac factor or antihemophiliac globulin.