The word azulejo (tile) comes from the Arab al zulaycha or zuléija, meaning a little polished stone.
A tile is a thin slab or plaque of fired clay, glazed and painted on one side, usually square in shape and of varying thicknesses and dimensions. Portuguese tiles are traditionally 14 x 14 cm. Some of the most important buildings in Portugal, such as royal and noble palaces, as well as churches and convents are faced with tiles, that sometimes completely cover the interior.
This original use began in the 16th century, and by the 19th century it had been transformed into huge revetments covering the façades of buildings, and it has been continued with great effect up to the present day on Metro stations in Lisbon, the Oceanarium and other public institutions.
（Museu Nacional do Azulejo、国立アズレージョ美術館の展示より）