There are a few different thresholds, and the quality of the carbs (glycemic index / glycemic load) also matters.
Generally a good starting point is the Zone Diet macro split: 40% carb, 30% protein, 30% fats, by calories. On a 2000 cal/day diet, that's roughly 800 calories (or 200g) carbs. Low carb is generally anything less than that, with < 100g being fairly seriously constrained, and ketogenesis generally requiring ~50-100g.
And yes, most unprocessed diets are reasonably low in carbs, or at least have slow carbs. Though there are some relatively high-carb indigenous diets, particularly among Pacific Islanders eating manioc and breadfruit, though fish and seafood are also staples.
Generally a good starting point is the Zone Diet macro split: 40% carb, 30% protein, 30% fats, by calories. On a 2000 cal/day diet, that's roughly 800 calories (or 200g) carbs. Low carb is generally anything less than that, with < 100g being fairly seriously constrained, and ketogenesis generally requiring ~50-100g.
And yes, most unprocessed diets are reasonably low in carbs, or at least have slow carbs. Though there are some relatively high-carb indigenous diets, particularly among Pacific Islanders eating manioc and breadfruit, though fish and seafood are also staples.
Good overview of "essential carbs": http://www.ajcn.org/content/75/5/951.2.full