I see the term "premium font" used a lot, but cannot find a definition of the term. What makes a premium font different from, I presume, non-premium fonts?
Premium is simply a marketing term but in the case of fonts its generally understood to mean a font you have to pay for. Its not that simple as not everyone uses this term so there are "fonts" you have to pay for as well. But generally a font foundry isn't the one calling their fonts premium. Some blogger author is trying to get people to their website by telling you what the best premium fonts are, for example:
"Feel free to submit any premium/commercial font..."
"Free fonts are all well and good, but because the entry point is $0.00, there are a few issues that arise: mainly, quality can be substandard and overuse of the same well-designed fonts can be rampant. One solution? Buck up and pay for some beautifully made premium fonts."
Its not quite what Scott said in that its not so much about the person selling the font. Its generally some author writing about the font that decided to use this terminology.