There is some difference between British
usage; in the UK, "road bike" refers exclusively to a road racing
bike by contrast with touring bike
s, track bike
bikes and the like, also usable on the road; in the USA it is used more in contrast with "mountain bike
" to refer to any type of bike not
designed for off-road use. Only a matter of nuance, though.
The design of bikes for road racing is fairly tightly prescribed by the UCI in order to preserve the primacy of the athlete over the machine as a factor in racing; this was, somewhat controversially, made more restrictive from 1999 onwards. Frames must be made from tubes in the traditional diamond pattern, wheels must be spoked and identical in diameter, no handlebar extensions are permitted, and restrictions have been put on various frame design parameters in order to prevent radical changes in the riding position.
The term is also used (in the UK at least) to denote a motorcycle
designed for normal road use as distinct from specialist racing bikes and the various types of off-road machine (trials, trail, enduro and speedway bikes, etc.), even if the latter happen to be road legal