Are they defined anywhere officially? Like is there a standard which says how much volume or weight is held by one teaspoon or one cup? Because I have teaspoons which vary widely in sizes, the same with cups. And even if you use a teaspoon to measure - is it with the contents flat? Or in a little pile? It's too ambiguous, and therefore - not a unit of measure.
It's obvious this is some kind of pet peeve, but a little googling would answer your own questions. It seems like the NIST is the regulatory body in the US presiding over official measurements, and they provide a brief overview here:
http://www.nist.gov/pml/wmd/metric/cooking.cfm
You're welcome to do more research to confirm Wikipedia's assertions. As for whether it's flat or a little pile, 1 tsp is flat (5ml). What you're thinking of is the commonly used cooking instruction "heaping tablespoon", because yes, when cooking you don't have to have everything down to the exact ml.
