An extended group of words or a phrase that extends a keyword, URL or domain name to make it more profitable.
Long-tail keywords and/or long-tail URLs can yield higher advertising returns because they are more specifically targeted and consequently higher converting, as well as less expensive because of less competition.
Long-tail domain names can also be used to target specific markets, as well as overcome name availability limitations for saturated domain extensions (e.g., .com, .net, etc).