Ruben Dahm and his colleagues call for greater “flood resilience” in delta cities (Nature 516, 329; 2014). But achieving resilience depends on what we mean by it: there are more than 70 definitions in the scientific literature.

These definitions vary between two extremes, with most trying to achieve a balance between the two (M. de Bruijne et al. in Designing Resilience 13–32; Univ. Pittsburgh Press, 2010). Dahm et al. implicitly define resilience as the ability of a system to bounce back after stress, as do many politicians and the World Economic Forum in Geneva, Switzerland.

At the other extreme, resilience is seen as “the capacity of social–ecological systems to adapt or transform in response to unfamiliar, unexpected and extreme shocks” as proposed by ecologist Stephen Carpenter and economics Nobel laureate Kenneth Arrow, among others (S. R. Carpenter et al. Sustainability 4, 3248–3259; 2012).

To recover, to adapt or to invest in both possibilities? All make sense in the right context. We might want a city to recover from flooding, for example, or the world to adapt to the effects of climate change.

Long-term policies to promote either recovery or adaptation, or prepare for both, are likely to be very different. We must have a clear definition from the outset.