Ping is a slang term that typically refers to a spell, hero power, or card effect that deals a small amount of damage (usually exactly 1) to a specific target. The name comes from the low amount of damage "pinging" off of something. The most straightforward ping is the Mage basic Hero Power . As such, Fireblast is usually referred to as "ping". Pinging can be considered the minion counterpart to Reach, as it usually extends the player's damage.
Pinging is a powerful tool in minion trading. It lets the player deal with 1 health minions and Divine Shield without wasting resources. It's important after trading into a sticky minion like , cleanly dealing with the it summons. Pinging can also be done to your own minions, such as activating an Enrage effect or drawing a card with .
The possibility to ping also adds flexibility to player's options by helping reach break points; whereas a player holding a could remove a 3 health minion, a player with as well can efficiently deal with a 4 health minion. However, unlike a player with , they retain the option of killing a 1 or 3 health minion more efficiently. A combination of small removal is more flexible than having one big removal card, but comes at the cost of having to use more cards.
Area of effect cards can also be pings when played against a single minion, or played to specifically deal with a single minion. Clearing a board of 1 health minions with a card like can also be called pinging the board.
Whether or not something is a ping depends on the availability of that effect. For example, is usually called a ping because Shaman has a hard time dealing 1 damage to a minion. Therefore, it's a common play to use it to deal 1 damage to a specific target. However, is rarely called a ping since Mage can typically use Fireblast instead.