Omnipresent describes something that's everywhere at once, like a deity. If your parents are constantly monitoring what you do, you may feel like they're omnipresent —and look forward to going to college somewhere far, far away.
You might recognize the prefix omni-, meaning "all," from words like omnipotent ("all-powerful") or omniscient ("all-knowing"). Add it to present, and voilà, you have a word for something that's present all the time. You can't hide from an omnipresent being, and you can't escape an omnipresent smell. Language purists might prefer that a bad smell be described as ubiquitous ("turning up everywhere") and save the word omnipresent for the divine.