The term 'broadband' is used to describe any form of communications
channel which is capable of tranmitting/recieving over a predetemined number of bits per second
This number varies from country to country, and is usually dependant upon the smallest frame size of the countries telephone systems that client traffic is multiplexed up to, called the primary multiplexers, and even then tends to vary.
In the UK, we primarily use SDH technology, so the broadband magic number is determined by the size of a VC-12 frame, which is 2.048 Mbits/s. In the US the predominant telephonic carrier system is SONET, which is very similar to SDH but uses different frame sizes, hence the usual definition of broadband is determined by the size of a DS1 frame, which is 1.544 Mbits/s