単純
たんじゅん · tanjun · JLPT N3 · na-adj
simplicity
単純 = single + pure: one thing, nothing mixed in, so it is simple
単 (tan) means single, as in 単語 (word) and 単位 (unit). 純 (jun) means pure or unmixed, as in 純粋 (purity). Something made of a single, unmixed part is simple. As an adjective, 単純な means simple or plain.