One of the key elements in establishing a legal claim to another person's property using adverse possession doctrine is that the trespasser's possession was "actual." In this world of virtual reality, does "actual" have a significant legal meaning? It does, but fortunately, "actual" possession is perhaps the most basic, intuitive element in a claim for adverse possession.
A person claiming real property adversely must be in actual possession of the property in order to make a claim effective. The person must have possession in the sense of an occupation of the land. They must, to put it simply, be there, both live and in person. This is distinguishable from presumed or hypothetical possession.
Consider this example. Suzy lives across the street from a vacant lot. That lot technically belongs to Bob, who lives across the street as well. In the three decades that Bob and Suzy have lived in the neighborhood, Bob has never used the lot.
Suzy is an avid gardener, but her own backyard is fairly small. She would love to be able to have ownership of Bob's lot, so that she could plant larger flowers and vegetables. When she first moved in, she asked Bob if she could use it, but he refused.
So she came up with an alternate plan. Every morning, she wakes up and looks at the lot out her window, mentally planning what types of trees and plants she will put in, and where. She visits the lot once or twice, mostly to measure it and clear out old tires and trash.
Assuming Suzy has met the statutory period in her state, can she establish adverse possession over the lot?
Unfortunately, she cannot. She has no actual possession over the land in question. She's done nothing to put the world, including Bob, on notice of her claim except to visit it a couple of times. Looking out the window at a piece of property, even with the intent to own and improve it, is not enough to establish ownership via adverse possession.
What could someone like Suzy do, if she wanted to try to establish title over that lot through adverse possession?
She'll need to cross the street, regularly, and begin actually planting her flowers and her vegetables. She might enclose the area with a fence, or she might begin moving her gardening equipment there and leaving it overnight. She might build a shed on the lot to store that equipment. She should regularly visit the lot, every week if not every day, to water the plants, prune the bushes, and care for the land. She might even put up signs that say, "PRIVATE GARDEN – NO TRESPASSING."
All of these acts would constitute actual possession of the land, within the meaning of the adverse possession requirements. Note that they would also constitute open and notorious possession, another necessary element of an adverse possession claim. With both elements, the purpose is to put the true owner on notice that someone is making a claim to his land.
Here, Bob would have every reason to be aware that Suzy is now squatting on his property. If Bob cares about his rights, he can easily file an action for ejectment or call the police. If Bob has gotten used to the idea of Suzy being there, but doesn't want to lose ownership, he can grant written permission for her use, and perhaps charge her rent. Or if Bob doesn't realize that he owns that vacant lot, or has stopped caring whether Suzy takes it, then so be it; Suzy can establish her ownership after the statutory period. But her actual possession of the land is a crucial first step.