Ambush

Ambush is a Passive Ability keyword, normally present on Units.

It is functionally short for "I may be played to a battlefield where you control Units" and "I have [Reaction] as long as I'm being played to a battlefield where you control Units."

It is a passive ability that adds options to locations that are valid for a Unit to be played to during the Make Relevant Choices step of Playing a Card.

Ambush can also appear as a verb on a card. In such a case, the verb is taken to mean "play with the permissions of the Ambush keyword."

If there are no units at the location chosen before Finalization completes for any reason, then it is no longer a valid location by Ambush's reasoning and the card cannot be played there.

Multiple instances of Ambush are redundant.

Ambush, and whether or not a unit has Ambush, is a characteristic of the Unit and may be checked or referenced by other Game Effects.