Answer: the contribution an individual makes to the gene pool of the next generation relative to the contribution of another individual.
Explanation:
It is the ability for an organism to reproduce and create viable offspring (offspring live to maturity and which can reproduce themselves). Relative fitness is the survival rate relative to the maximum survival rate with other population.
For example if organism A ends up finding a mate and have babies and organism B does not this means that organism A has more fitness biologically than organism B