This was a post on my Facebook page earlier today. I felt it was good to share here, too.
This subject came up on a recent writing group post, and I thought some additional clarification here might be nice.
Answer: Let's say I'm writing a chapter in which the POV belongs to my character Aeryn (Into the Darkness, The Joy Thief). She and Theo are sitting in the common room of an inn. Aeryn is watching an argument nearby.
Theo, who was minding her own business a moment before, comments on the argument. I type: "Theo followed her gaze."
How does Aeryn know this?
Unless I specified that Aeryn looked over at Theo when she spoke, Aeryn's focused on watching the argument. I just head-hopped.
What isn't head-hopping?
If your piece is written as third person omniscient, it's perfectly fine for you to jump from one person's thoughts to another's.
Why? The narrator is all-knowing. The narrator can bounce into any head that they want. Joyce Carol Oates did this in her story "Mastiff".