subadditivity

Noun

 * 1)  The state of being subadditive.
 * 2) The statement that a function is subadditive.
 * An outer measure $$\mu^*$$ satisfies a property called “subadditivity”, viz.: $$\mu^*\big(\bigcup_{j=1}^\infty A_j\big) \le \sum_{j=1}^\infty \mu^*(A_j)$$. If the less-than-or-equal sign were replaced with an equal sign then the property would be “additivity” instead.