Carbonated sweet water with minimal calories. Diet sodas may be beneficial to a diet due to satiety effects and displacing sugared sodas, and their health effects should be seen as a sum of the parts like Energy Drinks. Not inherently healthy or unhealthy.
Diet Soda is most often used for
It does not inhibit fat loss at all, and may actually suppress appetite that could help fat loss (although it does not induce fat loss per se either). Diet soda, in regards to body fat, is a carbonated inert beverage.
Research suggests that diet soda is unlikely to be detrimental to metabolic health or body composition. Further controlled trial evidence is needed to explore other possible detriments.