口訣

mnemonic chant / rhyme for remembering (arithmetic tables, character stroke order etc)

mouth / classifier for things with mouths (people, domestic animals, cannons, wells etc) / classifier for bites or mouthfuls

to bid farewell / tricks of the trade / pithy mnemonic formula (e.g. Mao Zedong's 16-character mantra 十六字訣|十六字诀 on guerrilla warfare)