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Carrying capacity is typically defined as the maximum population size that can be supported indefinitely by a given environment, but there are at least 10 different specific definitions used in basic and applied ecology. The concept originated in applied ecology with a focus on maximizing production of cattle on rangeland, and independently evolved in basic ecology associated with the logistic population growth model. Further definitions in basic ecology include (1) the equilibrial population size or density where the birth rate equals the death rate, (2) a long-term average population size that is stable through time (regardless of varying demographic rates), and (3) population size constrained by whatever resource is in the shortest supply, in accordance with the law of the minimum. Additional definitions in applied ecology include maximum sustainable yield, maximum economic yield, and the open-access equilibrium. In both basic and applied ecology, carrying capacity has proven difficult to parametrize and measure, rendering the concept to be most useful as a heuristic and theoretical tool. Nonetheless, it is a fundamental truth that no population can grow without limit, which is especially important given that many human societies have behaved as if no limits exist.