A Ghost Heart.
Ghost Hearts (GH for short) are hearts that appear in Luigi's Mansion. When the player flashes or shines the light on a ghost, it will be stunned and frozen then it will show its heart for a limited time. Then Luigi has his chance to inhale the ghost up with the Poltergust 3000, however it can only be done when its ghost heart is showing.
The Portrait Ghosts are more intense to catch as they will not shine their hearts so easily and quickly. The player needs to find their weakness and then the player can shine the light on the ghost and suck them into the vacuum. The player, however, needs only eighteen out of twenty-three portrait ghosts to save Mario, while the other five are optional. These optional ghosts, however, will give the player a lot of treasure if they manage to catch them in the Poltergust 3000.