A transfusion is a medical procedure where blood is transferred from one person into another person's body. When someone loses a lot of blood from an injury or surgery, or when their blood can't do its job properly because of illness, doctors can give them blood donated by healthy volunteers. The donated blood flows through a tube into a vein, usually in the arm, replacing what was lost or helping the body work better.

Blood transfusions save countless lives every year. Before doctors understood how to do them safely, people who lost too much blood would often die. Now hospitals keep supplies of donated blood ready for emergencies. Not everyone's blood works the same way, though. People have different blood types (like O, A, B, and AB), and doctors must match them carefully. If you receive the wrong type, your body can reject it.

You might also hear transfusion used metaphorically, like when someone talks about a “transfusion of new ideas” into an old project, meaning fresh thinking that revitalizes it. But the medical meaning is by far the most common. When you see blood donation drives at schools or community centers, they're collecting blood for transfusions that might one day save someone's life.