In optimization (a branch of mathematics), a candidate solution is a member of a set of possible solutions to a given problem. A candidate solution does not have to be a likely or reasonable solution to the problem – it is simply in the set that satisfies all constraints.
The space of all candidate solutions is called the feasible region, feasible set, search space, or solution space.
In the case of the genetic algorithm, the candidate solutions are the individuals in the population being evolved by the algorithm.