Gaslamp fantasy, also called gaslight fantasy, is Steampunk's more magically-inclined cousin. It's a subgenre of fantasy (and more specifically of Historical Fantasy) with a setting that is clearly recognizable as the real-world 19th or very early 20th century (or a reasonable analogue thereof). That's the Regency period and the Victorian and Edwardian eras, if the work's set in England, which it usually, though not necessarily, is. Victorian London is especially popular.