tokens: It was only in the second half of the 20th century that the term came to be used with any frequency , now mostly referring to the cultural flourishing of science and mathematics under the caliphates during the 9th to 11th centuries ( between the establishment of organised scholarship in the House of Wisdom and the beginning of the crusades ) , but often extended to include part of the late 8th or the 12th to early 13th centuries .