A blood clot was a solidified mass of blood that formed in a person, typically from injuries. When a clot ended up in a major organ, it could cut off blood flow and end up being fatal.
In the 22nd century, Miranda Cleaves developed "a very deep parietal clot". Her ganger also possessed this condition which manifested in the form of headaches in both Cleaves and would inevitably become fatal. In the 22nd century, Cleaves' clot was inoperable on Earth. Though Cleaves had seen doctors about her headaches before a solar storm brought the Gangers to life, she had not yet received the test results. After scanning Cleaves with his sonic screwdriver, the Ganger Doctor was able to diagnose the problem. After the Ganger Doctor stated that it was inoperable on Earth, Cleaves realised that he was not who he claimed. She later revealed the truth to her ganger self. After the crisis was over, Cleaves asked the Eleventh Doctor about her clot and the Doctor provided her with clot dissolving medicine from his TARDIS that would dissolve the clot. (TV: The Almost People)