Maybe overpopulation refers to an upper limit on population density. Yet some of the most densely packed areas on Earth are also some of the most economically successful, while some parts of the world with very low population density ratios are also some of the poorest, most starvation-prone. Or perhaps overpopulation occurs when population growth outstrips available resources. Some cultures in the past did apparently outgrow their resource bases, but the past few centuries have been marked by enormous economic growth which vastly exceeded the increase in population. Yes, world population increased dramatically during this century, but the world's standard of living increased even faster.
Overpopulation is, in fact, a subjective evaluation.
There is no optimal population level encoded in the DNA structure of human beings. As such, overpopulation will always entail human judgments and value-laden statements. It is my position that overpopulation should be defined as a situation where it is impossible to provide everyone on Earth a living standard at the subsistence level. If that is indeed a good measure, then not only is the world not overpopulated now, but it is extremely unlikely, based on long-term trends, that it will ever be overpopulated in the future. Because it contains several features other attempts at defining overpopulation fail to provide.