Elemental Damage refers to 
Fire, 
Cold, and 
Lightning damage.
Note that elemental damage bonuses come in two varieties that function differently.
- Percentage-based bonuses ("+n% Elemental Damage") add the percentage to each of the three types individually. If your percentage bonus is +50%, and your elemental damage without it is 50 Fire, 30 Cold, and 20 Lightning, with the bonus you would deal 50 × 1.5 = 75 Fire, 30 × 1.5 = 45 Cold, and 20 × 1.5 = 30 Lightning damage.
- Flat elemental damage ("n Elemental Damage") is divided equally. If your flat bonus is 60, and without the bonus, you would deal 50 Fire, 30 Cold, and 20 Lightning damage, after the bonus you would deal 50 + (60 / 3) = 70 Fire, 30 + (60 / 3) = 50 Cold, and 20 + (60 / 3) = 40 Lightning damage.