Romance Writers of America defines Historicals as "Romance novels set in any time period prior to 1945, and taking place in any location." That's a pretty broad definition and encompasses every time period and setting, from Medieval Scotland to the American West to the Tang Dynasty (800s China).
Good authors provide historical accuracy without overwhelming the story or the reader. There are often crossovers with Romantic Suspense and increasingly Paranormal Romance.
Chasing spies in France or jewel thieves in England, can lead these lords and ladies to True Love.
Different times, different places, but always in search of True Love.