I am very new to investing and I am particularly interested in fixed income product. I am just wondering what do people mean when they say US 10 year Government Bond? This term confuses me as I can think of a number of securities that can be referred by this term:

1. a bond issued today and matures in 10 years
2. a bond issued 10 years ago and have a 20 year maturity
3. a bond issued 20 years ago and have a 30 year maturity

Each of this bond will have a different risk characteristic and thus different coupon rate and yield. So when people say US 10 year yield, what are they really talking about?