What is “purgatory” in medieval Christian belief?

The story behind the answer

In medieval Christian thought, some who died in God’s friendship but still bore lesser sins or unpaid temporal penalties did not proceed straight to heaven. Instead, they experienced a remedial, post-mortem cleansing that perfected them for the beatific vision. Because its purpose is healing and sanctification, not retribution, it differs from hell’s unending punishment and from any purely earthly moral effort.

The doctrine matured in the Latin West in the 12th-13th centuries, taught by scholastics, depicted famously by Dante’s mountain in the Divine Comedy, and affirmed by councils at Lyon (1274) and Florence (1439). It fueled medieval practices like prayers and Masses for the dead, indulgences, and founding chantries—acts believed to aid those being purified. The Reformers later disputed it, but it remained central to Catholic belief.