Summoning sickness

Summoning sickness is an informal term for the rule that a creature cannot attack or use activated abilities either with the tap (undefined) or untap symbol if it has not been continuously controlled by a player since the beginning of that player's most recent turn.

Description
Summoning Sickness is what a creature has directly after it is cast onto the battlefield from a player's hand, graveyard, exile, and command zone; and means that the creature is neither able to attack nor use its tap ability that turn. The idea behind the term is that a creature is so disoriented by the experience of being summoned that it has to rest before it can do anything more than defend itself or use simple abilities.

Creatures that have Haste do not suffer from the effects of summoning sickness and can be attackers as soon as they are cast onto the battlefield.

Summoning Sickness doesn't stop you from using an ability on a creature. Summoning Sickness doesn't stop you from using the creature as a blocker.

Rules
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Usage on cards
The term actually appeared on cards from Mirage through to Urza's Legacy, where it was used in the phrase "{cardname} is unaffected by summoning sickness", with the same meaning as the modern keyword ability haste (for example, see Viashino Sandscout). Summoning sickness stopped appearing on cards in Sixth Edition when the Haste keyword was introduced. However, as the term summoning sickness is highly flavorful, had been widely popularised through appearing on cards, and because the rules no longer gave a proper term for the effect, summoning sickness still remains in use as a colloquial expression.

In recent times, the term "summoning sickness" is seen on the Future Sight card Dryad Arbor, which is a land creature, though only in reminder text, which does not have to be a "rules-tight" explanation. In Zendikar Rising, the term is also used on the card Ashaya, Soul of the Wild, reminding that nontoken creatures also become lands and follow the rulings of both types (They’re still affected by summoning sickness). Because of the latter use, Mark Rosewater would no longer call the term “informal”.