Betterment is defined under American law as "an improvement of real property which adds to its value otherwise than by mere repairs."
Improvement is defined as "a betterment; that by which the value or excellence of a thing is enhanced; a beneficial or valuable change or addition. An improvement in real property is something done or added to it, which increases its value, as cultivation, or the erection of or addition to buildings."
The following definitions are taken from the rules of the Interstate Commerce Commission:
"Additions include additional structures, facilities, or equipment, not taking the place of anything previously existing."
"Betterments include the enlargement or improvement of existing structures, facilities, or equipment, or a proper proportion of the cost of new structures, facilities, or equipment of an improved or higher class, taking the place of others previously existing."