Definition of MNEMONIC. 1. : assisting or intended to assist memory; also: of or relating to mnemonics. 2.: of or relating to memory. — mne·mon·i·cal·ly \-ni-k(ə-)lē\ adverb.See mnemonic defined for kids.. : assisting or intended to assist memory; also: of or relating to mnemonics. 2. : of or relating to memory.