[consp]

It used to be based on relative size, so if you have a set of spoons and cups and use the same for all measurements they are ballpark right for your recipe (and some minor difference accounting for user error). These day's it's defined anyway in both metric and imperial. As soon as you start weighing something from the recipe it goes out of the window as that defines the rest of the relative measurements. For that reason I really dislike the recipes telling you to measure teaspoons of spices but grams or ounces of flour. I don't have two sets of measurement cups available. These days most cooking sites mention both though.

On a sidenote: an ounce is 100g here and a pound 500g. Mainly by being in common usage and translated to common used weights. "An ounce more okay?" is an easy way to sell more without mentioning how much it actually is in numbers.

[samus]

The validity of relative measurements in recipes starts to break down as soon as eggs are in play, which are not easily subdivided. On the other hand, that rarely matters and most recipes are fine with up to one more or less egg.