"Unique Visitors" is usually defined per a time period: daily, monthly, or session (generally 30 minutes). Without further details, if I had to guess, I'd say monthly, but I'm just a random stranger on the Internet, what do I know?

This is based on a monthly trends. Unique visitors are the different IPs+User Agents hitting over a one month period. One unique visitor is an IP+User Agent. It’s possible that same eyeballs are using different clients, so the number maybe a little inflated due to that.