From the paper on Search Based Procedural Content Generation:
Search-based procedural content generation (SBPCG) is a special case of the generate-and-test approach to PCG, with the following qualifications:
The test function does not simply accept or reject the candidate content, but grades it using one or a vector of real numbers. Such a test function is sometimes called a fitness function and the grade it assigns to the content its fitness.
Generating new candidate content is contingent upon the fitness assigned to previously evaluated content instances; in this way the aim is to produce new content with higher fitness.
PCG Wiki References
Paper on Search Based Procedural Content Generation.