What does "microwave safe" mean?

Does the phrase “microwave safe” have a specific meaning? If so, what is it exactly?

A little background: my wife and I have a set of four small stoneware plates that, while labeled “dishwasher and microwave safe”, get ridiculously hot when placed in the microwave. This weekend we had the family over to the house for my step-daughter’s birthday. She had grabbed one of these plates to warm up a couple slices of pizza. The plate was in the microwave for less than a minute. When she went to take it out it was so hot that she burned her hand and just about dropped the plate on the floor. The pizza itself was barely warm. Five minutes later, the plate was still too hot to touch. This sparked a debate among family members of just what “microwave safe” really means. I always thought it meant that it wouldn’t melt or spark or disintegrate if used in the microwave. Others agreed with my step-daughter that something “microwave safe” shouldn’t get so friggin’ hot in thirty seconds that you can’t touch it without getting a blister.

Generally, it means only that there’s no metallic trim that could lead to sparking in a microwave.

As you say, many dishes and cups marked as “microwave safe” are ill-suited for use in a microwave.

I always took it to mean that, as said, there was no metal, and also that it was strong enough to withstand the microwave. I’ve had certain plastic utensils melt in the dishwasher, and I imagine certain things could easily turn to goo in the microwave.

It’s means the dish won’t ruin the microwave, and the microwave won’t destroy the dish. Some microwave safe dishes are made as a function to heat up hot in the microwave.

The meaning of the phrase depends on the material used, but in general it means:
The object won’t melt or explode, and won’t leach toxic byproducts into the food.

Here’s a good description for various materials: link

missed the edit window, sigh, wanted to add:

Note that an “oven-safe” pyrex casserole dish won’t explode or melt if you take it out of a 450 degree oven - but there sure is no expectation at all that you could do so with your bare hands.

Here are the instructions from the Kenmore microwave oven that’s in my apartment.

Leaving the issue of metals aside, microwave ovens work because they excite the water molecule. Things with a high water/moisture content heat faster. That is why microwaves don’t seem to really heat glassware very much. Glass has a very low or no moisture content. The food gets hot and the dish stays cool, except for the heat transfered from the cooking food to the glass.

With bakeware, clayware and ceramics, the moisture content of the actual plate or bowl may be enough to cause it to crack or explode if microwaved. So ‘microwave safe’ baking ware is just material that has a low enough moisture content to not crack. But it still may have enough moisture to cause the dish to get hot.

It seems there are two different definitions of “microwave safe”. You can find numerous references to DIY tests to determine the microwave safeness of an unmarked item by putting it in the microwave with a cup of water and zapping it for a minute. If the item gets warm, it is deemed not microwave safe. Apparently this is not the definition used by manufacturers, but I am with Shoeless in that I assumed that it was.

Apparently, in the case of one kind of bowl I owned, ‘microwave safe’ means ‘will seriously crack if used to hold a potato being cooked in the microwave.’