Synergy

From MTG Salvation Wiki

The amount of synergy in a deck is based on the overall network of card interaction within all parts of the deck.

[edit] Timed synergy

Some synergy may be connected to the decks Mana curve where the turn to drop certain cards adds to the overall synergy. It may be obvious that you must play a goblin before you play a Goblin Grenade, but sometimes cards have to be cast at a certain order. The more strict such a play order becomes the more synergy will be lost in the process. The optimal goal of synergy is to cast any card time and in any order and still get a bonus from other cards interacting with the card being cast.