Patanjali, the compiler of the Yoga Sutras, defines yoga as the cessation of fluctuations of the mind. This definition is found in the second verse of the first chapter of the Yoga Sutras, known as the Samadhi Pada. Patanjali states, "Yoga chitta vritti nirodhah," which can be translated as "Yoga is the stilling of the fluctuations of the mind."

This definition of yoga is significant because it highlights the central focus of the practice - the mind. According to Patanjali, the state of yoga is achieved when the mind is free from distractions, disturbances, and fluctuations. In other words, when the mind is calm, clear, and focused, one can experience the true essence of yoga.

Patanjali further elaborates on this definition by explaining the various types of fluctuations that can occur in the mind. He categorizes these fluctuations into five types, known as vrittis. These vrittis are pramana (correct knowledge), viparyaya (misconception), vikalpa (imagination), nidra (sleep), and smriti (memory). Patanjali states that these vrittis are the root cause of suffering and prevent us from experiencing our true nature, which is pure consciousness.

To overcome these fluctuations and achieve the state of yoga, Patanjali outlines the eight limbs of yoga, known as Ashtanga Yoga. These eight limbs are yama (restraints), niyama (observances), asana (physical postures), pranayama (breath control), pratyahara (withdrawal of the senses), dharana (concentration), dhyana (meditation), and samadhi (absorption). These limbs serve as a practical guide for individuals to achieve a state of yoga by purifying the mind and body and ultimately reaching a state of stillness and oneness.

Patanjali's definition of yoga is not limited to physical postures or exercises but encompasses all aspects of life. He emphasizes the importance of controlling the mind and freeing it from distractions to reach a state of inner peace and harmony. This definition of yoga is still relevant today, and practitioners of yoga continue to strive towards achieving a state of yoga through the eight limbs.

In conclusion, Patanjali defines yoga as the stilling of the mind's fluctuations, which can be achieved through the practice of the eight limbs of yoga. His definition highlights the importance of the mind in the practice of yoga and serves as a guide for individuals to reach a state of inner peace and harmony.