The definition of fragility (and its inverse antifragility) is the acceleration of harm. For example, if you plot speed of a glass cup hitting the floor and amount of harm to it, the curve rapidly accelerates as the speed goes up. Fragile things are harmed by disorder and stress.
Therefore the definition of antifragility is the opposite—things that improve as disorder and stress grows. For example, natural selection results in an ecosystem that is more resilient as disorder increases (more species, more variability resulting in better fitness, etc.).
See also:
- Thinking in systems shows how multiple stocks has a stabilizing effect on the overall system
- Antifragile systems exhibit a compounding effect for example, open source software that exponentially increases in popularity/value as it adds more contributors