An alcohol based fluid that is miscible in water and used to remove heat from the engine. Also see the [cooling-system] tag for related components such as the radiator and water-pump.
Simple And Technical Definition Of Coolant
From a technician's standpoint "engine coolant" would be referring to the 50/50 mix of ethylene/polypropylene glycol and water. From an engineers standpoint engine coolant would technically be anything that assists the system in the cooling process. So, in this case you could refer to the fans, oil, and Anti-freeze (The 50/50 mix) as "coolant". By definition coolant is any liquid or gas that can be utilized to maintain a desired temperature for any situation needed. Technically "air" is a gas of sorts. Fans moving air in a manner that would cool down your engine bay, could be considered a coolant. Not to get to technical, but you asked =)
If your average Joe says coolant, they mean the Anti-freeze (50/50 Mix stated above).
In almost every car made from the early 60's, you have pumps for the propulsion of various fluids. The two main pumps are "water pumps" and "oil pumps". The water pump moves the coolant throughout the engine block, head, and air conditioning heating system. The oil pump moves oil throughout the engine block and head to provide lubrication, cleaning and sometimes pressure to certain internal components that may need it. These systems typically do "Loop". The most common return for oil passages are held within the head. The oil pools up just enough to reach holes that take it back to the bottom of the engine. Coolant typically runs on a circuit as well. How these are routed is very specific to the engine manufacturer. 99 percent of the time you have one oil pump and one water pump.
I will say that your engine Anti-freeze does the majority of work. The reason is because the rate at which it circulates is far quicker than oil. It also has its own cooling assistance (The radiator). The entire system by nature is designed to handle the brunt of the job.
Oil does its part as well, but in a bit of a different way. Oil keeps the system cool by minimizing friction. The less friction, the less heat. As a fluid substance it has a good amount of potential to release heat, so it does that as well.
Air as explained above carries its purposes. Most manufactures use the grill or face of the car to route air into the radiator and engine bay. They also use air spoilers to route air into the engine bay. The ambient temperature at which an engine "lives" is actually very impacting on how well the engine can stay at its desired temperature. Not all engines do this though, it depends on the rate at which the engine turns and it's stresses. Hope this helps you understand a bit better, and I hope I answered your question well enough =)