Presence is the state of being somewhere. When you get an invitation that reads "Your presence is requested,” you are being asked to show up. Your style of being there — your demeanor or bearing — is also your presence.
Usually, presence is a physical thing. Since the 1600s, the word has also meant something otherwordly, a spiritual or divine presence. If you suddenly feel a weird presence in the room, perhaps it's a ghost . . . or just the cat. Use the phrase "presence of mind" to indicate when someone is quick-witted or just paying attention. While your job washing skyscraper windows doesn't require a lot of brain work, it's a good idea to keep your presence of mind while working.