Direct Fitness or Darwinian Fitness is a central tenet of the Theory of Evolution. It is the measure of an individual organism's ability to reproduce. Direct fitness is measured as the proportion of genes a given individual contributes to the next generation due to its direct reproductive efforts. This can be contrasted with inclusive fitness, which also includes the indirect fitness measure of genes contributed to the next generation based on aid given to relatives that are not direct descendants.
Misconceptions about fitness
Fitness is not simply "survival". Rather, it is the number of viable offspring produced by an organism. In other words, fitness (both direct and indirect) can be measured by the number of copies of a gene which are passed on to subsequent generations. Looking at the sheer number of offspring can be misleading, as it ignores how successful those new organisms are.
It is possible for a gene to increase fitness yet actually harm an individual. Such a gene would function by making a host organism more fertile but more susceptible to dying at a younger age. In biological terms, such an organism would still have more fitness.
Fitness and genes
Thinking of fitness at the genetic level can simplify this somewhat; a gene has one goal: to create more copies of itself. Thus, the gene is successful if it creates copies which are in turn successful at creating more copies. As the gene, rather than the individual, is the focus of this idea, it avoids the common errors associated with fitness. It also simplifies the apparent dichotomy between direct and indirect fitness: it does not matter what body the gene is residing in. All that matters is how many copies of the gene are created.
Campbell, N., Reece, J., et al. 2002. Biology. 6th ed. San Francisco, California. pp. 1145-1148.
Richard Dawkins; The Selfish Gene; Oxford University Press; Oxford, England; 1976, 2006