Is Ducat an Action or a Treasure, and why does its text lack the words when you play this?

Question

Accepted Answer

Ducat is a Treasure, which is why it is played in the Buy phase. The words "when you play this" that other Treasures carry are only a reminder and are not needed; Ducat's +1 Buy and +1 Coffers still happen when it is played.

Dominion: Intrigue

Is Ducat an Action or a Treasure, and why does its text lack the words when you play this?