The nirodha state of mind is not only in the definition of yoga: this state of mind is also the goal of yoga. A nirodha mind is fully controlled and restrained, which can be attained through meditation and contemplation practices.
According to a commentary of the Yoga Sutras by ancient Hindu author, Vyasa, nirodha is the fifth and most desired of the five states of mind. It is a mastered mind with full control and, if a yogi can sustain it for a long period of time, he/she can realize the true Self and reach the state of final liberation, or moksha.
The controlled mind should not be confused with suppressed thoughts. The thoughts and emotions stay, but, with deep meditation, the mind becomes still and one-pointed. This mastery of deep stillness is called nirodha.