Unfortunately, the only definition is an empirical one: containers that don’t get hot in the microwave are microwave safe. Remember (or go look it up now in “The World Around Us”) that microwaves heat food by causing molecules with dipole moments to tumble back and forth. If the container has any such molecules, it’ll get hot in the microwave. If any water has leached into your ceramic mug, it’ll get hot in the microwave. Regardless of whether the container gets hot on its own, the food you are heating up will get hot and transfer that heat to the bowl, so be careful when taking hot things out of the microwave.