Answer:
When a liquid is used as a medium to transfer heat to heat a system, it is called hydronic.
Step-by-step explanation:
Solar collector is used to collect the radiation (heat) coming from the sun Heat from the sun is collected and is used to heat a substance like water.
When a liquid is used to heat or cool a system, it is said to be hydronic. Hydro is related to water. Any liquid system can be called hydronic. Water, glycol, liquid mineral water etc are used as the liquid heat transfer medium.
To summarize, when a liquid based system is used to transfer heat to heat a system, it is called hydronic. Hydro is a term related to water.