Hmm vs. Umm: What's the Difference?

Hmm and Umm Definitions

Indicating thinking or pondering.

Hmm... that's an interesting idea.

Hmm... I just can't decide between the two.

Hmm... creation or evolution? That’s the big question.

Used to express doubt or uncertainty or to fill a pause when hesitating in speaking.

A demand for an answer to a question.

Just where were you until 2 a.m.? Hmm?

Implying hesitation.

- How does a plane fly?

- Umm, I don't know.

(intransitive) To make a thoughtful humming noise.

To make the umm sound to express confusion or hesitancy.