A retroist is one who looks to the past to find their current style. Examples can be found in the 1960s feel of Belle & Sebastian, the 1970s feel of The Thrills, or even in the 1980s feel of The Darkness. Not solely applying to music, however, it a can also describe a person who attempts to utilize vintage clothing to define their style.
Retroism is by no means a new phenomenon, but a relatively new term. One can see some of the early trends of retroism in programs like Happy Days or even in the Edwardian sensibilities of early BBC programming. Retroism is the attempt to bring an anachronism of an earlier modern period into a contemporary work of art, style of fashion, or even lifestyle.
Essentially a retroist is a period revivalist of a modern period, typically a decade.