General guidelines on the quantitative representation of heterogeneity in ecology are still lacking. Without a precise definition and sound quantification of heterogeneity, statements involving the concept will continue to be confusing, and comparative studies and hypothesis testing will be ineffective. To solve this problem we propose an operational definition of ecological heterogeneity that facilitates quantification based on the data types of concern. We use two examples to demonstrate how our definition and quantification of spatial heterogeneity can be applied in practice.