The term preadaptation is applied when a large change in function is accomplished with little change in structure.
As we look at the historical changes in the evolution of an adaptation, it is possible that at every stage the organ served the same function (the evolution of the eye is probably an example - all its stages probably had visual functions), or the earlier stages could possibly have had different functions from the later ones. The classical Darwinian term for the second possibility is preadaptation. The earlier stage is described as a preadaptation for the later stage.
The evolution of tetrapod limbs is an example, illustrated by an animation. A structure which was effective at locomotion in water was also good at locomotion over land with relatively few changes.
Preadaptation does not imply any futuristic or anticipatory faculty in evolution. All that is happening is that sometimes, by chance, an organ that works well in one function turns out to work well in another function after relatively little adjustment.
Image: it has been suggested that bird feathers are an example of a preadaptation: the first feathers were for heat insulation rather than an adaptation for flight.