This is what the government said in the 1964 Wilderness Act:

"(c) 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. An area of wilderness is further defined to mean in this Act an area of undeveloped Federal land retaining its primeval character and influence, without permanent improvements or human habitation, which is protected and managed so as to preserve its natural conditions and which (1) generally appears to have been affected primarily by the forces of nature, with the imprint of man's work substantially unnoticeable; (2) has outstanding opportunities for solitude or a primitive and unconfined type of recreation; (3) has at least five thousand acres of land or is of sufficient size as to make practicable its preservation and use in an unimpaired condition; and (4) may also contain ecological, geological, or other features of scientific, educational, scenic, or historical value."

In general, if I go out far enough so I can't get back at night and I don't see people except close to the trailhead, that's wilderness to me.

It's really difficult in Western Colorado not to have a bailout point each day. It's quite easy in eastern Colorado to have 40 miles between places to get water.

As for what it's like? There is a lot less to think about than there is when urban backpacking. It's a lot simpler. Off for an urban hike.