When liquid starts to thicken and become solid, it coagulates. When you get a cut, the blood flowing from the wound will coagulate: it will start to clot and form a solid scab so you will stop bleeding.
Many liquids have the potential to coagulate. If the cream you just poured into your coffee is spoiled, you'll see the cream coagulate as it curdles into little floating chunks. If someone has a heart condition that may result in a heart attack, he or she might take medication that keeps the blood in the arteries from coagulating, or dangerously thickening.