John Wilkins wrote in his blog:

“Fitness is a property of a competing variant in a population. It means that X, whatever it might be biologically, is increasing in its frequency in a population faster than its competing variants. X can be a gene, or a trait, or even an entire organism’s form and functionality.

What fitness isn’t, is something absolute. There’s no universal measure of fitness that applies to all organisms. Every different kind of biological variant is only fit or not compared to the other variants in its population, whether that’s a local population, or a gene pool, or even a smaller group like a kin group (an extended family). Fitness is relative, literally and figuratively.”

… and, in one of his own blog-follow-up comments:

“… I think that fitness is a rate of frequency change, is that it provides a momentum view of fitness - it’s the rate right now at which the frequency of that X is changing, irrespective of what happened in the past or will happen in the future. This is how I get over the generation issue.