Dash

Dash is a keyword ability on permanents that allows the player to play the card for an alternative cost. When you pay its dash cost instead of paying its mana cost, the creature will have haste, so it can attack that turn. At the beginning of the next end step, you'll return the creature from the battlefield to its owner's hand.

Description
Dash was introduced in Fate Reforged where it replaced Raid from Khans of Tarkir as the mechanic associated with the Mardu Horde. It also features in Dragons of Tarkir, associated with Clan Kolaghan. Without the keyword, the mechanic was featured as early as Visions, on Viashino Sandstalker.

Dash costs range from being one mana cheaper to one mana more expensive than the converted mana cost, largely contingent on context. The typical trend is that Dash costs pull towards the middle of the mana curve into the 2-3 mana spot: cheaper ones use Dash as superior utility late in the game, while expensive ones use Dash as a bonus when missing a spot on the curve. Those that defy this trend have high impact attack triggers, or in the case of Pitiless Horde, use it as downside mitigation.

Dash also appeared in Modern Horizons.

Rulings

 * If you choose to pay the dash cost, you're still casting the spell, so it goes on the stack and can be countered.
 * Dash doesn't change when you can cast creature spells — usually only during your main phase.
 * If you cast a creature spell using its dash cost, it will return to your hand only if it's still on the battlefield at the beginning of the next end step. If it leaves the battlefield before that point, it'll just stay wherever it is.
 * You don't have to attack with the creature with dash unless another ability says you do.
 * If a creature enters the battlefield as a copy of or becomes a copy of a creature whose dash cost was paid, the copy won't have haste and won't be returned to its owner's hand.