My question arose from the following example.

Hero Mablung reads: Response: After you engage an enemy, add 1 resource to Mablung's resource pool. (Limit once per phase.)

Attachment Heir of Mardil reads: Response: After attached hero gains any number of resources from a card effect, exhaust Heir of Mardil to ready attached hero.

Is the response in Heir of Mardil considered a card effect or a response? Would Malblung be readied in this example if he gets a resource token from his own response ability?

Is there a definition for what qualifies as card effect? Is it basically anything that comes from a card?

It's pretty much everything that comes from a card that isn't already covered by the standard rules. For instance, dealing damage from an attack exhausting to commit to a quest aren't card effects. There are also some subtleties with the scope of a card effect. For example, Quick Strike's card effect causes the target to make an attack. This is a card effect. However, the attack itself is not a card effect, so can be used to attack "immune to player card effects" enemies.

Note however that Heir of Mardil can only be attached to Noble heroes, so to get it on Mablung you also need the Diligent Noble card or similar trickery.

NathanH is right about your case: every effect on a card that give resource work for heir of mardil. Mablung work, steward of Gondor work. I would say that neclace of girion/resourceful work too. But sword-thain didn't work since, even if it make you have one resource more, only rule give a resource to the new heroes created by the effect, not the card effect itself.

But his quick strike example is wrong: quick strike target the enemy so we can't choose an enemy with "immune to player card effects". The rest of the post is both right and clear so good job anyway ;).

Quick Strike does work on immune enemies per the FAQ:

Q: Can I use Quick Strike (Core 35) to declare an attack against an enemy that is immune to player card effects? A: Yes. The effect of Quick Strike targets a character and allows that character to make an attack, therefore the chosen enemy's immunity does not factor.

Thanks everyone!

A card effect is any effect . . . on a card! Which is pretty much every effect in the game, depending on your definition of the word "card."

18 hours ago, Wandalf the Gizzard said:

A card effect is any effect . . . on a card! Which is pretty much every effect in the game, depending on your definition of the word "card."

It is everything but the effect of the rules.
Example: having a character destroyed because he have as many or more damage than life point is cause by rules, not card effect. Progress placed thanks to a quest success are placed by the rules, not by card effects, etc...