Cantrip

Cantrip is the common slang term for any Dominion Kingdom card which offers +1 Action and +1 Card. It is essentially self-replacing (costing no net action to play it) and will usually offer some side benefit, such as the + Peddler offers or the +1 Action of Village.

The cantrip name is a recycled slang term from the Magic: The Gathering card game, which itself borrowed the term from Dungeons and Dragons.

Cantrips harming the deck

In most cases, cantrips are seen as cards that cannot harm the deck. There are a few exceptions.

Discard attacks like Militia or Goons which require the player to make a choice of which cards to discard are more powerful against hands containing a few cantrips. This is because the player discarding does not know which cards are going to be drawn by the cantrips, so the chance of making a sub-optimal choice is greater.

Cantrips also anti-synergize with draw-up-to-X cards like Library, Watchtower, and Jack of all Trades. Although cantrips do not outright harm the deck in these cases, they under-perform other cards which do not draw cards (and which typically offer more benefits in compensation for the lack of draw).