Perceive the presence of ghosts and spirits
The first example of Mediumship was in "Dead Man Dating", where Piper and Phoebe could see the ghost of Mark Chao while he was invisible to everyone else around him. The Charmed Ones have been able to see earthbound spirits many times after this and can also see their loved ones as spirits when summoned.
Gypsies are also seen exhibiting this ability in a different form. In "The Eyes Have It", Gypsy Shuvani Ava Nicolae was able to receive a message from her recently deceased aunt Lydia. Gypsies call these appearances a mulo.