The term thrombosed means an obstruction of the flow of blood within a blood vessel that is caused by the clotting of coagulated blood within the blood vessel. The term coagulated means turned from a liquid into a semi-solid or a solid.
So, a thrombosed hemorrhoid is a hemorrhoid where the blood has clotted and dried, forming an obstruction within the blood vessel. As you can see, this obstruction further constricts blood flow compounding the problem.
Often the overall effect is to tear the blood vessels open. This causes a visible lump directly on the anus. Even if the hemorrhoid is internal (which normally cannot be seen on the exterior of the anus) in this case, the hemorrhoid will often prolapse or protrude to the outside of the anus and is therefore visible.
Thrombosed hemorrhoids are usually hard to the touch and appear as a dark purple lump. They are often prone to bleeding as the blood vessels rupture from the pressure of restricted blood flow.
Sometimes a thrombosed hemorrhoid can break apart and heal on their own. This is especially true of one keeps it clean and watches their diet closely, especially eating fiber.
Occasionally, thrombosed hemorrhoids may be severe enough to warrant the need for surgery. In these cases one might consider the consultation of a medical physician.