What is a convection loop

With respect to what, a water heater, a heating system or a stove?

It is a term used in HVAC and plumbing. Basically a loop system. In a house, hot air rises to the upper level, cools and then falls back to ground level. In older heating systems it was called a gravity system rather than forced air. In heated water systems, without a pump, water heated by a boiler will rise, gives up its heat and then returns. Modern systems use a pump to circulate. In solar houses, a heater wall will convect heat up to a higher level, cool and then return.

I like ma0641 answer. Well explained...

In addition: You can see convention hot water loops in older houses. You could actually attach hot water circulating pump to it and turn it into regular hot water circulation loop systems we use today.