A mnemonic is a phrase or sentence devised to help people memorise groups and / or sequences which they would ordinarily have trouble remembering. Probably the best known example of a mnemonic is the one for remembering the colours of a rainbow in their correct order, thus :-
Richard Of York Gave Battle In Vain
Red Orange Yellow Green Blue Indigo Violet
Richard Of York Gave Battle In Vain
Red Orange Yellow Green Blue Indigo Violet