Cantrip is the common colloquial term for any Dominion Action card which offers +1 Action and +1 Card. It is essentially self-replacing (not taking up space in your hand or taking up an action) and will usually offer some side benefit, such as the topdecking effect Harbinger offers or the additional Favor of Underling.
In the narrowest definition, a cantrip offers exactly +Card and +1 Action, and no extra amount of +Card, +Actions, or +. In a broader sense, the term cantrips encompasses all those cards that also fall into another category, such as non-terminal draw (if the card draws more than one card), village (if it provides more than +1 Action), or Peddler variant (if it also offers +).
In most cases, cantrips are seen as cards that cannot harm the deck, since you receive a benefit by playing them and doing so does not prevent or delay you from drawing other cards in your 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.
Similarly, although cantrips don't "take up space" in your deck on your turn since you can draw right past them, there may be cards you want to have in your hand between turns—Reactions to protect you from an opponent's Attack, Province to block an opponent's Tournament, etc. Filling your deck with cantrips will make it less likely that you have in hand the card you need when it's not your turn. (Cards other than cantrips have this property as well, of course; but it's easier to lose sight of the fact with cantrips since they don't have this property on your turn.)
Like any other Action card, a cantrip can be drawn dead—i.e., drawn by a terminal Action card without sufficient +Actions to be able to play the cantrip after you draw it—and thus can harm a Big Money deck that depends on terminal draw without villages.
Cantrips may also be less useful 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).
 List of Cantrip Cards
Cards in italics have been removed. Cards with an asterisk (*) were added in the second edition.
- Base set: Harbinger*, Sentry*
- Intrigue: Mill*, Pawn
- Alchemy: Familiar
- Hinterlands: Highway, Scheme, Trail*, Wheelwright*
- Dark Ages: Cartographer, Market Square, Sage, Urchin, Sir Bailey
- Guilds: Herald
- Cornucopia and Guilds: Farrier
- Adventures: Page, Ratcatcher, Guide
- Empires: Settlers, Groundskeeper
- Nocturne: Secret Cave, Pixie
- Renaissance: Border Guard
- Menagerie: Sanctuary, Groom, Way of the Pig
- Allies: Town Crier, Blacksmith, Old Map, Underling, Innkeeper, Miller, Hill Fort, Sorcerer, Sorceress, Swap
Enchantress causes the first Action each other player plays on their turn to become a cantrip.
Any village that draws a card can be considered a cantrip. For a list of villages, see here. Any card that non-terminally draws multiple cards can be considered a cantrip. For a list of those cards, see here. Any Peddler variant is also a cantrip, for a list, see here. Some can cards can act as non-terminal draw, as a village, or as a Peddler variant under specific circumstances, but act as a simple cantrip otherwise:
- Wishing Well
- Sea Chart
- Scrying Pool
- Battle Plan
- Cabin Boy
 Removed cards
 Fringe cases
- Vassal can act like a cantrip if the top card of your deck is a non-terminal Action.
- Ironworks becomes a cantrip if it is used to gain an Action–Victory card.
- Mystic is a cantrip if you can guess what card is on top of your deck.
- Spice Merchant is a Copper trasher with a net effect resembling a cantrip: after you trash a Copper from your hand, it can give you +2 Cards and +1 Action, leaving you with the same handsize and action count you had before you played it.
- Forum, Fugitive, and Secret Passage draw more than 1 card, but maintain handsize. The same also applies to Tide Pools if its entire effect over both turns is considered.
- Raze acts as a sifting cantrip if you trash a card costing more than .
- Goatherd is a cantrip if the player to your right trashed at least one card during their previous turn.
- Royal Galley draws and doesn't give +Action, but does itself play an Action from your hand.
 Handsize-decreasing cantrips
These cards provide +1 Card, +1 Action, but mandatorily decrease handsize by trashing, discarding, or setting aside a card from the hand.
 Simple cantrips
 Can act as village, non-terminal draw, or peddler sometimes
 Handsize decreasing cantrips
The name "cantrip" is a slang term from the Magic: The Gathering card game, where it describes a spell that draws a replacement spell on casting. Magic borrowed the term from Dungeons and Dragons, which ultimately derived it from the misty depths of Scottish etymology.
Some players refer to Cantrips as 'invisible' cards, based on them replacing themselves into your hand with no net negatives. Donald X has referred to Cantrips as 'free' cards.