I hear you asking this question and I have a very simple answer.
Antifreeze IS coolant, short and simple. Coolant is simply a term used in liquid cooling applications. Your car has a liquid cooling system designed to keep the engine operating within normal operating temperatures. Antifreeze is used in this application as a coolant with freeze prevention additives, hence the name "Antifreeze".
A more complicated answer is "what Antifreeze is". Antifreeze is an additive for water that lowers the freezing point and raises the boiling point of the water making it a more efficient coolant than just plain old water. Antifreeze has other additives like silicates and phosphates to combat corrosion, and distilled water lowers the conductivity of the liquid. Most automotive antifreeze is Ethylene Glycol with some applications utilizing Propylene Glycol instead. Propylene Glycol is considered to be non-toxic antifreeze, but isn't widely used in automotive applications.
What have we learned:
Antifreeze Is Coolant
Antifreeze is a water additive
Antifreeze raises the boiling point of water
Antifreeze lowers the freezing point of water
Not all Antifreeze is the same
ANTIFREEZE IS TOXIC AND SHOULD BE KEPT AWAY FROM CHILDREN AND ANIMALS