There is no exact definition of computational finance.
Basically, it refers to researches which use advanced computing techniques, such
as
computational intelligence, to studying problems in economics and finance.
One area of research in computational finance is to use
genetic programming
(a branch of evolutionary computation, which borrows its ideas from
natural selection)
to financial forecasting.
Another area of research is to build models of financial markets to enable policy makers or
banks to ask "what if" questions; this allows one to design new market mechanisms
(as one would wind-tunnel test aircraft designs).
For more information, see Tsang and Martinez-Jaramillo's
paper
in IEEE CISNewsletterVol2, No.3, August 2004.