心肌 (xīn jī) literally means 'heart muscle' — combining 心 (xīn), meaning 'heart', and 肌 (jī), meaning 'muscle'. It refers specifically to the specialized, involuntary striated muscle tissue that makes up the walls of the heart and enables it to contract rhythmically and pump blood throughout the body. Unlike skeletal muscle, cardiac muscle is self-excitable and highly resistant to fatigue.

This term is used almost exclusively in medical, biological, or health-related contexts — for example, when discussing heart disease, exercise physiology, or autonomic nervous system regulation. It rarely appears in everyday conversation unless someone is describing a health condition or studying biology. Because it denotes a precise anatomical structure, 心肌 is typically found in compound terms like 心肌梗死 (xīn jī gěng sǐ, 'myocardial infarction') or 心肌炎 (xīn jī yán, 'myocarditis').