The ecological validity of a sensory cue in perception is the correlation between the cue (something an organism might be able to measure from the proximal stimulus) and a property of the world (some aspect of the distal stimulus). For example, the color of a banana is a cue that indicates whether the banana is ripe. This particular cue has an ecological validity close to 1, because a banana's ripeness is highly correlated with its color. By contrast, the presence of a sticker on the banana is a cue with an ecological validity close to 0, if (as seems likely) ripe and unripe bananas (in a fruit bowl, say) are equally likely to have stickers on them.
The concept of ecological validity is closely related to likelihood in Bayesian statistical inference and to cue validity in statistics.
History of terminology
Egon Brunswik defined the term "ecological validity" in the 1940s to describe a cue's informativeness. His students have written that the now-common use of "ecological validity" to describe a type of experimental validity was a corruption of his original terminology (see external link to paper by Hammond). Brunswik used the words representative design to describe what is now usually called the external validity of an experiment; this in turn depends partly on what is now usually called the ecological validity of the experiment. As originally defined by Brunswik, however, ecological validity was a property of a cue, not a property of an experiment.