Domain

Domain is a term that refers to cards that rely on and count the number of different basic land types (one to five) a player controls. Domain cards were introduced in Invasion. In Conflux, Domain became an ability word.

Description
Domain was concepted by Barry Reich for a never-released set called Spectral Chaos.

In the Invasion block, there are a total of 20 Domain cards: 11 from Invasion, 7 from Planeshift, and 2 from Apocalypse. In addition, Conflux contains a total of 10 Domain cards. As a throwback to the past, there is 1 card with Domain in Time Spiral. All Invasion block Domain cards have been issued errata to have Domain as an ability word.

Domain cards encourage you to play five-color decks containing at least one of each basic land type. The Sunburst keyword from Fifth Dawn, which likewise encourages five-color deck building, can be seen as a latter-day cousin to the Domain mechanic.

Rulings (from Conflux)

 * Each domain ability has a different effect. Read each card carefully.
 * To determine the number of basic land types among lands you control, look at the lands you have in play and ask yourself whether the subtypes Plains, Island, Swamp, Mountain, and Forest appear within that group. The number of times you say yes (topping out at five) tells you how powerful your domain abilities will be.
 * How many lands you control of a particular basic land type is irrelevant to a domain ability, as long as that number is greater than zero. As far as domain is concerned, ten Forests is the same as one Forest.
 * A number of nonbasic lands have basic land types. These include five Shadowmoor nonbasic lands, the Revised Edition "dual lands", and the Ravnica block "dual lands", among others. Domain abilities don't count the number of lands you control—they count the number of basic land types among lands you control, even if that means checking the same land twice. For example, if you control a Tundra, an Overgrown Tomb, and a Madblind Mountain, you'll have a Plains, Island, Swamp, Mountain, and Forest among the lands you control. Your domain abilities will be maxed out.
 * A domain ability that appears on an instant, sorcery, or activated ability checks the number of basic land types among lands you control just once, as that part of its effect is applied. This value won't change even if which lands you control changes later in the turn. For example, if you control a Swamp and two Forests at the time Drag Down resolves, the targeted creature will get -2/-2 for the rest of the turn. If you play an Island later in the turn, Drag Down's effect won't change.
 * A domain ability that appears on a static ability continually checks the number of basic land types among lands you control. If that number changes, the effect of the static ability will change accordingly. For example, if you control a Swamp and two Forests when Matca Rioters comes into play, Matca Rioters will be 2/2. If you play an Island later, Matca Rioters will become 3/3.

Pre-ability word example
In the above example, if you have all 5 basic land types then the enchanted creature gets +5/+5.

Anti-domain cards
Two cards are designed to be effective against domain decks:
 * Mask of Intolerance</c> from Apocalypse
 * Planar Overlay</c> from Planeshift

Trivia

 * In design, R&D called Domain “the Barry mechanic” after its creator.
 * Domain was featured as rules card 2 of 4 in the Conflux set.