The FCC recently redefined it to be 25 megabits/second down / 3 megabits/second up.
One bit is either a 1 or a 0. A megabit is 106 bits. Not to be confused with a megabyte, which is 106 bytes -- a byte is (usually) 8 bits and what your file system usually shows file sizes in.
So broadband is, at minimum, the ability to download a 3.125 megabyte file in one second, or upload a 0.375 megabyte file in one second. Or at least have the capacity to do so if the server on the other end will let you.