chitta (Sanskrit: “memory”;) — derived from the root chit, “to be conscious”. Chitta is the Subconscious mind. It is the mind-stuff. It is the store-house of memory. Samskaras or impressions of actions are imbedded here. It is one of the four parts of antahkarana.
Patañjali defines yoga (in Yoga-Sutra I.2) as chitta-vritti-nirodha (the cessation of mental fluctuations). Vyasa’s commentary on the first sutra in the Samadhi-pada explains that chitta (the thinking substance or principle) has five stages: 1) the restless (ksipta), 2) the torpid (mudha), 3) the distracted (viksipta), 4) the focused (ekagra), and 5) the restricted (niruddha). The first three stages (ksipta, mudha, and viksipta) are not classified as yoga, but the next two stages (ekagra and niruddha) are classified as yoga.