In optimization  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

Constraint (mathematics)

In mathematics, a constraint is a condition that a solution to an optimization problem must satisfy. There are two types of constraints: equality constraints and inequality constraints...

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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

Genetic algorithm

A genetic algorithm is a search heuristic that mimics the process of natural evolution. This heuristic is routinely used to generate useful solutions to optimization and search problems...

, the candidate solutions are the individuals in the population being evolved by the algorithm.