For questions about the medieval era (500-1500AD o.e.) as a whole.
The word is generally used to refer to the era roughly between the fall of Rome and the fall of Byzantium, separating the ancient world from the modern era, so roughly 500AD to 1500AD.
Sometimes the word is assumed to have dark age connotations. In this sense, especially outside a European context, it should be used with caution, as the era was a golden age for the many a civilization, such as the Muslim Arab Caliphate and the Tang and Song dynasties in what is today China.
For questions specifically about medieval Europe consider medieval-europe.