The definition of homesteading usually refers to a "pioneer" individual or family trying to live off the land and create a home for themselves in the wilderness.
Historically, those settlers who first inhabited non-European lands or areas without the society and culture they were used to were called homesteaders.
They became self-reliant out of necessity as pioneers in a new wilderness.
In the United States, the settlement process began at Jamestown and spread westward with the Homestead Act. This was an old law and tradition in US that said the settlers who could remain on land long enough could claim it as their own.
There are modern homesteaders who are extreme, living off the grid and raising all of their own food. I say anyone who makes any effort at being self-reliant can claim the title.
Here are some related concepts and terms that you will see used frequently by urban homesteaders.
There are some excellent videos and books available if you want to learn more about the modern homestead and urban farming.
Personally, my favorite one is Countryside & Small Stock Journal. I love the ideas people write in to share and learning about what others are doing.