Hi, Captain Jostero card is:

"Once per round, after an enemy ship that is not defending against an attack suffers damage or critical damage, you may perform an attack against that ship."

Is this mean the enemy ship:

1. suffers damage from other source than defending.

or

2. in an attack not defending and suffers damage?

If the 1. is true I can bomb the enemy ship with Seismic charges with Nym and then shoot on him with Captain Jo? (In the Activation phase)

Yes you can trigger Jostero with a bomb. This isn't the only ability that enables you to attack during the Activation phase so that isn't unusual. Something like the splash from a Harpoon missile would work too, you just aren't allowed to double up on a standard attack.

Other ways to trigger Jostero:

1. Assault Missiles splash damage
2. Various bombs other than Proton Bombs
3. Seismic Torpedo
4. Feedback Array

Things that DON'T trigger Jostero, because they deal damage cards , instead of making ships suffer damage :

1. Proton Bombs
2. Black Market Slicer Tools