In the context of property law, perpetual easement is used to describe the rights entitled to a landowner to make limited use of his neighbor's land, such as crossing it to reach his own property, according to Dictionary.com. The site also notes that perpetual means that something that is never ending or has no limit regarding time.
According to Wikipedia, there are different types of easements in the legal world. An easement can be either affirmative or negative. An affirmative easement deals with one allowing another to use their property for something. A negative easement restricts someone else from doing something on their own property that would normally be considered lawful.
Those involved in an easement can be servient or dominant, within the easement. Someone who is servient takes the downside of an easement, while the dominant estate is the one gaining something from the easement.
Easements can also be intended for public or private use, with a public easement allowing for anyone to use the specified property. Easements that are appurtenant are generally used to help the dominant estate, and easements that are in gross are helpful to an individual or larger entity.
A floating easement is an easement in which there are no or little restrictions on the right-of-way. A structural encroachment occurs when a piece of property crosses over a property border, and is considered by some to be an easement.