The Medieval period (also known as the Middle Ages) was a time period in Europe that was said to have lasted for 1000 years, though no one is sure when it really ended. Today, the Middle Ages are synonymous with Knights.
At Danbooru, this tag should be used for fashion pertaining to this era, such as Knight's armor, gowns with long sleeves, corsets, laced sandals, boots, and even cloaks.
This should not be confused with the Renaissance period, which is said to have come after/during the Late Middle Ages.