Factor V (pronounced "factor five") is a protein involved in clotting. Unlike many other proteins involved in clotting, Factor V is not an enzyme, but instead binds to other clotting factors in the presence of certain other enzymes.
A shortage of Factor V can lead to bleeding. There are also genetic mutations of the protein which can lead to diseases, such as Factor V Leiden.
The role of Factor V in clotting was not understood until well after the four other clotting factors were identified in 1905. The first paper to describe Factor V was not published until 1947 and the protein wasn't fully described until 1987.