"Some things benefit from shocks; they thrive and grow when exposed to volatility, randomness, disorder, and stressors and love adventure, risk, and uncertainty. Yet, in spite of the ubiquity of the phenomenon, there is no word for the exact opposite of fragile. Let us call it antifragile. Antifragility is beyond resilience or robustness. The resilient resists shocks and stays the same; the antifragile gets better". The phenomenon is well studied in medicine, where for example Wolff's law describes how bones grow stronger due to external load. Hormesis is an example of mild antifragility, where the stressor is a poisonous substance and the antifragile becomes better overall from a small dose of the stressor. This is different from robustness or resilience in that the antifragile system improves with, not withstands, stressors, where the stressors are neither too large or small. The larger point, according to Taleb, is that depriving systems of vital stressors is not necessarily a good thing and can be downright harmful.
More technically, Taleb defines antifragility as a nonlinear response: "Simply, antifragility is defined as a convex response to a stressor or source of harm (for some range of variation), leading to a positive sensitivity to increase in volatility (or variability, stress, dispersion of outcomes, or uncertainty, what is grouped under the designation "disorder cluster"). Likewise fragility is defined as a concave sensitivity to stressors, leading a negative sensitivity to increase in volatility. The relation between fragility, convexity and sensitivity to disorder is mathematical, obtained by theorem, not derived from empirical data mining or some historical narrative. It is a priori".