  Singular limit problems of reaction-diffusion systems have been studied in
cases where the effects of the reaction terms are very large compared with
those of the other terms. Such problems appear in literature in various fields
such as chemistry, ecology, biology, geology and approximation theory. In this
paper, we deal with the singular limit of a general reaction-diffusion system
including many problems in the literature. We formulate the problem, derive the
limit equation and establish a rigorous mathematical theory.
