Negativity is a tendency to be downbeat, disagreeable, and skeptical. It's a pessimistic attitude that always expects the worst.
Negative outcomes are bad outcomes, like losing a game, getting a disease, suffering an injury, or getting something stolen. Negativity is an attitude that always expects such negative things to happen. "I bet it'll rain during our picnic!" and "No one could ever love me!" are examples of negativity. "The world is getting worse and worse!" is negativity. Someone full of negativity is hard to cheer up, and they always expect the worst. The opposite of negativity is positivity.