Confucius's teaching is usually built from cumulative, layered discussion rather than single definitions; ren, benevolence, receives different partial descriptions in different conversations throughout the Analects rather than one settled statement. This passage is unusual because Tsze-kung asks for the opposite, one word that could govern a whole life, and Confucius answers with a single term, shu, reciprocity, then glosses it himself in the same breath rather than leaving the word to stand alone.
Mark Csikszentmihalyi's Stanford Encyclopedia discussion of Confucius's ethics treats shu as standing beside ren as one of the small set of concepts later Confucian tradition built its systematic ethics from, and the passage's own plain gloss, what you do not want done to yourself, do not do to others, became the version later compilers reached for whenever the tradition needed the whole teaching compressed into something quotable and teachable to a beginner. Zhu Xi's Song dynasty synthesis, which fixed the Analects inside the Four Books as the core curriculum of Confucian education for the better part of a millennium, treated this exchange as the moment the tradition's ethics were stated in their most transmissible form, the single word a student could carry when the fuller discussions of ren elsewhere in the text were not yet available to him.