The right to a specific use of another person's property.
The main types of easements are easements in gross, easement by express grant and easement by necessity.
An easement in gross is the type of easement granted to your local power company to maintain their power lines. This means that when power lines go down in your neighborhood and they need to cross your property to get to some of their equipment to fix the problem, they have the legal right to do so. This does NOT cover meter-reading. Their easement in gross does not apply to entering your property for any reason, only those specific to certain things, depending on who the easement is granted to; railroad companies, power companies, water companies.
An easement by express grant means simply that written into the title of a property is the right to access to a part of a neighbor's property, such as with a road that leads to your neighbor's house and continues onto yours. You are given access to the first part of the road that might technically belong to them.
An easement by necessity is when you can't be denied access to your property, for example, when you are landlocked, that is, your property is between other properties and not directly connected to a road, and so you are granted an easement from your property to the nearest road via the shortest distance possible, over whoever's property that happens to be.