1) Fading is a term used to describe a procedure for gradually changing a stimulus controlling an organism's performance to another stimulus. For example, consider a pigeon which pecks at a green key and not at a red one. If a cross is superimposed on the green key and the green colour is faded out, the new stimulus will control the bird's behaviour without the occurrence of any unreinforced pecking.

2) At first, the SA and the SD differ along at least two stimulus dimensions. The difference between the SA and the SD along all but one dimension is reduced until there is no difference along the reduced dimensions. Then the SA and the SD differ along only one dimension.