There is no definition of prime but here are some considerations -
1) How hard is it to get the card? A prime card does not have to be elite, but if everyone could get one, we probably wouldn't consider it prime. whogavemecredit.com is a decent indicator of that and there are websites that give recommended scores for cards (though they are believed to be inflated)
2) If it is a rewards card, how good are the rewards and does the card offer a conceirge service? Considering how good the rewards program is. Does the APR seem outrageous?
3) If it is a low interest rate card, how close to the prime rate is it?
4) Given a reasonable income - will the card give credit limit increases every 6 months to a year up to say 15k? There are cards with low rates that will never top a few thousand in CL's - and this is a killer to your utilization.
5) Is there an annual fee? A maintenance fee?
In essence a prime card is a card on terms that are favorable to the borrower when compared to the market average for that type of card. A prime card indicates a lender is willing to potentially lose thousands of dollars to you and believes the chance of your repayment is such that it need not charge a inflated interest rate to compensate for this risk of default.