Purified sweet compounds from the stevia plant, 200–400× sweeter than sugar.
Natural vs approved
Highly purified steviol glycosides are approved; whole-leaf and crude extracts are not approved as sweeteners in the US — a nice example that 'natural' isn't a safety category.
Evidence & sources
JECFA set an ADI for steviol glycosides of 4 mg/kg body weight per day (as steviol). Evidence: regulatory. Known: JECFA evaluation. Unknown: — Source: Joint FAO/WHO Expert Committee on Food Additives, Evaluation of steviol glycosides (2008).