Named after the goddess Mnemosyne, any technique or trick used to enable memory. For example, the words HOMES can be used to memorize the names of the Great Lakes:

Huron
Ontario
Michigan
Erie
Superior

The “m” is silent, so the word is pronounced “neh-mahn-ics.” Especially before books became widely available, the art of memory was considered highly important, and mnemonic techniques were of great value.