What is the test of time?

The followint quote is from an interesting post in another thread:

I don't think it will stand the test of time. Most music doesn't.

What exactly is the proverbial test of time though? Critics spout that phrase all the time. But what exactly qualifies something to have stood the test of time? Critics said that rap wouldn't last over twenty years ago. Critics said that pop wouldn't last over forty years ago. Critics said that r&b wouldn't last over fifty years ago. Critics said that swing wouldn’t last over seventy years ago...

Interesting subject. Since the dawn of recording technology and especially now, people everywhere are scrambling to archive recordings of the past into whatever is the latest, greatest medium and format. In light of this, just about everything ever recorded will survive time. But still the question remains, what is the "test" of time. If anyone on earth listens to something long forgotten, does that mean it has stood the test of time?

I'd love to hear your thoughts on the topic. What is the “test of time” to you?