1. Literally, outside one's door. I'm at your doorstep right now—where are you?

2. By extension, one's responsibility. That issue is at the police commissioner's doorstep now. I always hire reputable contractors for my company because I don't want complaints of shoddy construction at my doorstep.

3. Very close to one's location, especially of an unwanted thing that is encroaching. We used to think of it as a skirmish on the frontier, but the war is now at our doorstep. This whole area used to be rural, but with rampant development, the suburbs are at our doorstep.