Corporealization is the ability to become corporeal, thus assuming a (temporary) solid form.
List of Users
Spirits
Certain spirits, such as Penny Halliwell, when summoned through the To Summon the Dead spell, can assume a solid form after stepping out of the circle in which they are summoned. Other spirits, such as Rick and Nathan Lang have displayed the ability to become corporeal on their own by channeling their anger. Jackson Ward was able to pick up objects.
The Necromancer
The Necromancer, the ghost of a demon, could corporealize by absorbing other spirits. However, this effect was only temporary, depending on how strong the absorbed spirit was.
Valkyries
Valkyries search battlefields for the spirits of warrior and absorb their souls. Once they return to Valhalla and release the souls, the warriors will corporealize.