Is 'Om' considered a sound, a prayer, an incantation, or something else?

It’s a mantra. A word or sound repeated to aid concentration in meditation.

Om is the sound that created the universe. It is used as an aid for concentration and to praise the creator. It is at once the originator and encompasses all.

“om… (1788) : a mantra consisting of the sound \om\ and used in contemplation of ultimate reality” -from Merriam-Webster’s Collegiate Dictionary, 11th ed.

Depends on who’s listening.

From what I’ve learned, it’s all and each of those things.

I think the answer @Hawaii_Jake gives fits my own undestanding best.

And – without Om there’s no omlette.

or Omaha

It’s a chant.

^^ only if you say it more than once. hehehe

^^Which you will unless you require no air.