Reciprocal Link, Two-Way Links or more popular known as link exchange. It is a link building strategy with mutual agreement from two parties. A website creates a hyperlink to B website, and then B site returns a hyperlink to A website.
For initial stage this is a workable strategy, both website gain additional referral traffic and backlinks. When more backlinks point to a website, it help to higher up search engine rankings. Later the strategy rapidly practice by webmasters, some even offer link building services and create a link exchange communities for bigger link pool. As a result, reciprocal link has turn become a spam link activity and discourage by search engine.