[sin-er-jee]–noun, plural ~gies.
1. combined action or functioning; synergism.
2. the cooperative action of two or more muscles, nerves, or the like.
3. the cooperative action of two or more stimuli or drugs.
Let's take flixerflax's Cusre plus Elemental Strike example. Curse and Elemental Strike do not work together. Curse prepares the target making it more suseptible to Elemental Strike. It does nothing to improve Elemental Strike. Similarly, the Elemental Strike probably never even realises there's a Curse in place.
With flixerflax's Blizard plus Frost Orb example, the training from the two skills can be combined as they are on a similar topic. When casting Frost Orb, your training in Frost Orb and Blizard cooperate to provide additional benefits.
And to bring it to the real world, imagine two surgeons performing breast surgury. THey might decide that one works on the left and the other on the right. This is good time management, not synergy. If they combine their skills and work together on each breast, the whole operation may take longer, but the overall result is likely to be better. That's synergy. (And I only chose breast surgury because I can't think what else would commonly be performed on the left and right simultaneously.)