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.