The words defrost and thaw can used interchangeably, but there is a difference between the two. "Thawing" implies the process of allowing something frozen to return to its natural state as a result of increasing temperature.
This could be frozen food or any other product that has been froze.
Defrosting is the process of removing frost from a frozen food item. This can be done by placing the food item in the fridge, or by using a microwave.
it is importnat to remember that when it comes to frozen food, you might "thaw" a piece of chicken by leaving it out at room temperature, while you "defrost" your freezer to remove frosts.