Loosely translated, it refers to "laments", but the precise meaning varied over the centuries.
[1] It is most frequently used in reference to bu-date, division of waka anthologies into thematic books.
[1] The 9th-century Bunka Shūreishū (an anthology of poems in classical Chinese) used the word aishō (哀傷).
[1] This shift in usage continued in later collections, and by the time of the Shinkokin Wakashū the category included a very large number of poems on impermanence.
[1] The precise meaning and boundaries of what constitute an aishōka are not certain and seem to have fluctuated, specifically grown broader, over time.