This effect keyword first appeared in the Call of the Crystals set.
Ambush is a keyword passive ability (written in bold in the effect area of the card).

Definition:

A hero or ally with ambush will not receive any retaliatory damage when attacking an other ally or an other hero. In other words, the attacked hero or ally will not retaliate any combat damage back to the attacker.

Unidirectional Damage:

The main benefit of ambush is that it allows the attacker to deal damage without risking counterattacks from the attacked target. This makes it particularly valuable for allies with lower health who might not survive a regular combat exchange where they only damage the target.

Some statistics:

• Shadow faction has 4 ambush allies, Human faction has 6, and Neutral faction has 4.
• The Rogue class has 5 allies with ambush and three abilities that give ambush to allies, making it a key class for this ability.

The cards related to ambush are