To immobilize something means to make it unable to move or to prevent it from moving. When a doctor immobilizes a broken arm with a cast, the hard plaster keeps the bones perfectly still so they can heal properly. A spider immobilizes its prey by wrapping it tightly in silk. In tag, you might immobilize your friend by lightly touching them so they have to stop running.

Something immobilized is stuck, frozen in place, or held completely still.

Immobilize often suggests a deliberate action with a purpose. A veterinarian might immobilize a frightened animal gently so she can examine it safely. In stories, a wizard might immobilize an opponent with a freezing spell, or quicksand might immobilize an explorer who stepped in the wrong spot. Police officers sometimes use special techniques to immobilize someone dangerous without causing injury.

You can also immobilize objects: a car immobilizer is a security device that prevents the engine from starting. Fear can immobilize people too. When someone is so scared or shocked that they freeze and can't move, we say they're immobilized by fear, though this is temporary rather than physical.