In poker, a ‘scare card’ is a term used to refer to cards that potentially improves your opponent’s hand, which makes you wonder if you still have the best hand. It is a term that is generally used when over cards and flush cards hit on the turn or river.
EXAMPLE: The flop was 5c-6c-Ks. Any club on the turn or river would be a scare card because it may give someone a flush. Also, a 2, 3, and 9 might be considered scare cards because they complete an open ended straight draw.
Scare cards fall in different categories of scariness based on the likelihood that the card actually helped an opponent.
EXAMPLE: The player in UTG position opens to $12 and you decide to call with 98 suited on the button. The flop comes down 3h-4d-9s and villain cbets. You call. The turn is a blank and you both check. The turn is an Ah and villain bets. In this situation it’s a big scare card because most players are raising with a fairly tight range of hands in early position, which mainly consists of pairs and good aces like AK, AQ. It looks like his cbet was an attempt to steal the pot and he/she slowed down on the turn, and then fired when he/she caught up. Obviously there is a chance that the villain has nothing because scare cards provide good bluffing opportunities as betting scare cards can be profitable.