Perfect Max (PM) refers to a fused card (the rarity ends with EX) that is at maximum level with perfect stats.
The "perfect" part refers to the two fusion cards being at maximum level before fusing. Because they were both upgraded fully before fusing, they contribute 10% of their stats instead of 5%. Thus, the new EX card can attain the maximum possible attack and defense.
The "max" part simply refers to the EX card being fully upgraded, at maximum level. Once the card is maxed, any further upgrading will only have a chance of raising the skill level.
- Note: A few people consider a perfect max card to also have a skill level of 10. Though this is uncommon, a card with skill level 10 is worth much more than a card of skill level 1.
PM cards are usually double the price of the base card in Energy Packs or Power Packs, in addition to a small amount of EP or PP to compensate for the time, cards, and Credits spent on upgrading. However, it is becoming more and more common to see PM skill 1-3 cards selling for merely double the cost of the base card. (February 4, 2013)
When trading for PM cards, be aware that, currently, the only reliable way to tell if it is PM or not is if the card is level 1 or at max level. The fusion bonus is a flat value added to the base/leveled stats of a card, so, theoretically, if you knew what the stats should be for a card at a given level, you can calculate whether the bonus is correct or not.