'亦师亦友' (yì shī yì yǒu) is a four-character idiom meaning 'both teacher and friend' — describing someone who fulfills dual roles: offering guidance, knowledge, and mentorship like a teacher, while also sharing mutual respect, trust, and warmth like a close friend. The structure repeats '亦' (meaning 'also' or 'likewise') before both '师' (teacher) and '友' (friend), emphasizing the seamless coexistence of these two relationships without hierarchy or contradiction.

This phrase is commonly used to describe admired mentors — such as professors, senior colleagues, or experienced coaches — whose approachable demeanor and genuine care make learning feel collaborative rather than formal. It carries positive, respectful connotations and appears frequently in educational, professional, and personal reflection contexts, especially when expressing gratitude or admiration for someone’s balanced influence.