On this question on Engineering.SE a poster calls something 2" thick by 4" wide by 2ft long a "board". He asserts in a comment that "we call 2x4s "boards" here in the midwest of the U.S.". Personally I would probably call it either a "two-by-four" or a "stud". It is possible that in my native British English there isn't a generally accepted term for pieces of timber with a width to thickness ratio less than 4:1.
My research on dictionary.reference.com yields "board" as:
a piece of wood sawed thin, and of considerable length and breadth compared with the thickness
This agrees with my understanding of "board". Clearly a breadth of 4" is not "considerable" compared to a thickness of 2".
Merriam-Webster (the last word in US definitions as far as I understand) gives:
a piece of sawed lumber of little thickness and a length greatly exceeding its width
Although this gives no comparison of the thickness compared to the width, it does say "of little thickness" and doesn't mention that the width must be little. I would therefore interpret this to mean something like a floorboard: say 3/4" by 6" by 25".
Can anybody find any research to back up the poster's assertion that a 2"x4" is termed a board in parts of the US?