A wilderness is an area that is largely unaffected by human residency, agriculture or industry. Beyond that brief definition, deciding what is or is not a wilderness is somewhat subjective, like most of the finer things in life.
I was going to pawn off this question by citing the legal definition of wilderness, which is given in the United States by the Wilderness Act of 1964. I had assumed, before reading it, that a piece of folklore about distance from roads defined wilderness, but that is only part of the act.
Within ten years after September 3, 1964, the Secretary of the Interior shall review every roadless area of five thousand contiguous acres or more in the national parks, monuments and other units of the national park system and every such area of, and every roadless island within, the national wildlife refuges and game ranges, under his jurisdiction on September 3, 1964, and shall report to the President his recommendation as to the suitability or nonsuitability of each such area or island for preservation as wilderness.
With the suitability defined by
A wilderness, in contrast with those areas where man and his own works dominate the landscape, is hereby recognized as an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain.
which changes our subjective definition of "wilderness" to the more legally stringent "what the
Secretary of the Interior considers to be wilderness. Roadlessness is just a precondition. And of course, I would say that it doesn't take much investigation to realize that the trammeling that people can do to areas is not only of the obvious kind. Introduction of
invasive species,
alterations to the climate and the long standing practice of
fire suppression have all changed areas from what their "natural" appearance would be. Not to mention the fact that modern industrial society is always present in the hum of passing
jet planes.
And yet, legal definitions and uncertainty aside, I do believe in wilderness. There are certain places where the constant buzz of human culture, which is usually an invisible pressure, is suddenly absent. In the United States, the Wilderness Act seems to have done a pretty good job of deciding what those places were. Absence of roads and traffic, or other signs of human structures, is an obvious part of that. Other than that, it is based on a subjective feeling of being in a world where different rules apply. The fact that it is subjective does not mean that it is all a matter of gossamer---one of the biggest feelings I have in a wilderness area is the fact that if I hurt myself, I might die before anyone can help, and that I am actually at risk of being attacked by a predator.
Although the definition of what makes a wilderness is somewhat hazy, the definition of what does not make a wilderness, and where those non-wilderness areas are, is a little bit easier. Areas with obvious signs of development are not wilderness. There is very little wilderness in the eastern half of the United States (or in Canada, outside of northern Quebec and Newfoundland or in Western Europe. I find access to wilderness areas to be a part of my experience that may be hard to explain to people who do not have such access. I tend to lump together places like Wisconsin, Alabama and New York State based on the lack of wilderness in their geography, something that residents of those areas find somewhat puzzling.
Since wilderness is by definition, an area outside of human influence, at least seemingly, perhaps attempting to categorize it and define it is doomed to failure by definition. The wilderness is out there, something we can experience, and that is important to our experience, but not something we can ever quite define outside of experience.