In ecology, resilience has been defined in two competing fashions that emphasize two different aspects of stability.
The consequences of those different aspects for ecological systems were first emphasized by the Canadian ecologist C. S. Holling in order to draw attention to tradeoffs between efficiency on the one hand and persistence on the other, or between constancy and change, or between predictability and unpredictability. It is defined by the Resilience Alliance as "the capacity of an ecosystem to tolerate disturbance without collapsing into a qualitatively different state that is controlled by a different set of processes. A resilient ecosystem can withstand shocks and rebuild itself when necessary. Resilience in social systems has the added capacity of humans to anticipate and plan for the future." Resilence is conferred in human and ecological systems by adaptive capacity.
One definition of resilience is the rate at which a system returns to a single steady or cyclic state following a perturbation. This definition of resilience assumes that behavior of a system remains within the stable domain that contains this steady state.
When a system can reorganize, that is shift from one stability domain to another, a more relevant measure of ecosystem dynamics is ecological resilience. It is a measure of the amount of change or disruption that is required to transform a system from being maintained by one set of mutually reinforcing processes and structures to a different set of processes and structures.
The first definition focuses on efficiency, control, constancy, and predictability - all attributes at the core of desires for fail-safe design and optimal performance. The second focuses on persistence, adaptiveness, variability, and unpredictability - all attributes embraced and celebrated by those with an evolutionary or developmental perspective. The latter attributes are at the heart of understanding sustainability.
The first definition, which is more traditional, concentrates on stability near an equilibrium steady-state, where resistance to disturbance and speed of return to the equilibrium are used to measure the property. This type of resilience has been defined as engineering resilience.
The second definition emphasizes conditions far from any steady-states, where instabilities can flip a system into another regime of behavior - i.e. to another stability domain. In this case resilience is measured by the magnitude of disturbance that can be absorbed before the system changes its structure by changing the variables and processes that control behavior. This type of resilience has been defined as ecological resilience.
These two aspects of a system's stability have very different consequences for evaluating, understanding and managing complexity and change. Sustainable relationships between people and nature require an emphasis on ecological resilience, because the interplay between stabilizing and destabilizing properties is at the heart of present issues of development and the environment- global change, biodiversity loss, ecosystem restoration and sustainable development. Emphasis on engineering resilience reinforces the dangerous myth that the variability of natural systems can be effectively controlled, that the consequences are predictable and that sustained production is an attainable and sustainable goal.
The two contrasting aspects of stability- essentially one that focuses on maintaining efficiency of function (engineering resilience) vs. one that focuses on maintaining existence of function (ecological resilience)- are so fundamental that they can become alternative paradigms whose devotees reflect traditions of a discipline or of an attitude more than of a reality of nature.
Human (psychosocial) perspectives have also entered the interdisciplinary and cross-sector discourse on resilience as a dynamic process of adaptation and transformation in the face of adversity. Resilience is defined as a multidimensional contruct that refers to "the capacity of individuals, families, communities, and systems or institutions (formal and informal)to anticipate, withstand and/or judiciously engage with catastrophic events and/or experiences, actively making meaning of adversity with the goal of maintaining 'normal' function without fundamentally losing their identity." In this context, resilience is understood as much more than the absence of Post Traumatic Stress Disorder (PTSD), and measured, at least in part, using the "Sense of Coherence" (SOC) scale across scales and culture/language boundaries.