Before revision:  In economics, static stability is traditionally defined as a notion about how incentives respond to choices : for a population game, it means that, on average, the payoff of a strategy responds negatively to an increase in the share of the strategy.
After revision:  In economics, static stability is traditionally defined by negative relationship between endogenous and exogenous variables in a model : for a population game, it means that, on average, the payoff of a strategy responds negatively to an increase in the share of the strategy.