Low Carb

What is considered a low-carb diet? Is it not going over a certain number of grams of carbs per dat or per meal? Or is it a ratio of carbs per 100 calories?

I see things that have what seem like low carb like Fage plain yogurt that has 8 or 9 carbs per serving but I don't know if that's really low or just seems low in relation to something like pasta that has a lot of carbs.