Homesteading dates back to the "Homestead Act" and before. Like Commune or Rural living except for the isolation factor. Homesteader's were typically very isolated and had to live a life-style of subsistence and self-sufficiency.
Modern Homesteading is a life-style of subsistence and self-sufficiency to the degree that you are able do to where-ever you are living. Often growing and making things yourself instead of buying. It also includes recycling or upcycling used items. Basically leading as independent a lifestyle as possible by relying less and less on manufactured or processed goods.
The Modern Homesteading life-style overlaps a little with the Prepping life-style, Minimalism life-style and with the Off-Grid life-style.
The Backyard Farming Connection has the best description or definition that I can find.