When blood travels around the body, it moves through small tubes. An embolism is when something gets stuck in one of those tubes and stops the blood from going through. That something is often a hard lump of blood. Though actually, it does not have to be blood: a little air, a bit of fat, or another small piece can block the tube in the same way. The lump usually did not start in the place where it gets stuck. It formed somewhere else, broke away, and rode along in the blood until the tube became too narrow. Doctors have a name for the travelling piece (an embolus) and a name for the blockage (an embolism), but in ordinary talk people often treat the two as one idea. This is a hospital word, and a serious one. You will meet it in phrases like “pulmonary embolism” — that means the blockage is in the lung, which is dangerous because the lung cannot get the blood it needs. Friends do not use this word about a traffic jam or a blocked sink; that stretch sounds odd. The story of the word is a little sideways. Long before doctors used it this way, it meant putting extra days into a calendar so the year did not drift. That came from a Greek way of saying “throw in” or “insert.” In 1848 a doctor, Rudolf Virchow, took that picture and applied it to the body: the blocking thing is “thrown into” the blood. So the tidy modern meaning is medical, but the older meaning was about time.