In Christianity, the orb is a symbol of the power of the Pope.

In Hinduism, the orb is a symbol of the sun god Surya.

In Buddhism, the orb is a symbol of the Buddha's enlightenment.

In Native American cultures, the orb is a symbol of the Earth Mother.

--> In modern times, orbs are often seen as a sign of ghosts or other paranormal activity.

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I think you're talking about the last one.