739.1. Reaction is a Permissive keyword.
739.1.a. It can be present on Spells, Units, Rune Abilities, Legend Abilities and Permanent Abilities.
739.1.b. Reaction grants the corresponding card or effect all abilities and permissions of Action.
739.1.c. Reaction , additionally, is functionally short for the following:
739.1.c.1. On Spells: "This can be played during Closed States on any player's turn."
739.1.c.2. On Units: "This can be played during Closed States on any player's turn."
739.1.c.3. On Rune, Legend, or Permanent Abilities: "This can be activated during Closed States on any player's turn."