Is it ensure, insure, or assure?
"aiding the memory," 1753, back-formation from mnemonics, or from Greek mnemonikos "of or pertaining to memory," from mnemon (genitive mnemonos) "remembering, mindful," from memne "memory, a remembrance, record, an epitaph; memory as a mental faculty," from base of mnasthai "remember," from PIE root *men- "to think" (see mind (n.)). The noun meaning "mnemonic device" is from 1858. Related: Mnemonical (1660s).
mnemonics mne·mon·ics (nĭ-mŏn'ĭks)
A system to develop or improve the memory.
mnemonic mne·mon·ic (nĭ-mŏn'ĭk)
Relating to, assisting, or intended to assist the memory. n.
A device, such as a formula or rhyme, used as an aid in remembering.
any device for aiding the memory. Named for Mnemosyne, the goddess of memory in Greek mythology, mnemonics are also called memoria technica. The principle is to create in the mind an artificial structure that incorporates unfamiliar ideas or, especially, a series of dissociated ideas that by themselves are difficult to remember. Ideally, the structure is designed so that its parts are mutually suggestive. Grouping items in rhymed verse has long been a popular mnemonic technique, from the "gender rhymes" of the Latin grammars to the verse for remembering the number of days in the months ("Thirty days hath September, April, June and November").