Haste is a keyword for an ability which had been printed multiple times before.

702.10. Haste

• 702.10a. Haste is a static ability.
• 702.10b. If a creature has haste, it can attack even if it hasn't been controlled by its controller continuously since his or her most recent turn began. (See rule 302.6.)
• 702.10c. If a creature has haste, its controller can activate its activated abilities whose cost includes the tap symbol or the untap symbol even if that creature hasn't been controlled by that player continuously since his or her most recent turn began. (See rule 302.6.)
• 702.10d. Multiple instances of haste on the same creature are redundant.

Translation:

• 702.10a. - It's always on.
• 702.10b. - It is able to attack even if the controller hasn't had it for the most recent turn's beginning.
• 702.10c. - Activated abilities can be used even if the controller hasn't had it for the most recent turn's beginning.
• 702.10d. - Haste does not stack.