(Sanskrit) In Patanjali's Raja Yoga / Ashtanga Yoga, vrittis are modifications of mind, thought-waves.
"The modifications are five, [some] painful and [some] not painful: Right knowledge, wrong knowledge, fantasy, sleep, and memory." - Patanjali, Yoga Sutras 1:5-6
"Chitta is the mental substance. It takes various forms. These forms constitute Vrittis. It gets transformed (Parinama). These transformations or modifications are the thought-waves, whirlpools or Vrittis. If the Chitta thinks of a mango, the Vritti of a mango is formed in the lake of Chitta. This will subside and another Vritti will be formed when it thinks of milk. Countless Vrittis are rising and subsiding in the ocean of Chitta. These Vrittis cause restlessness of mind. Why do Vrittis arise from the Chitta? Because of Samskaras and Vasanas. If you annihilate all Vasanas, all Vrittis will subside by themselves. When a Vritti subsides it leaves a definite impression in the subconscious mind. It is known as Samskara or latent impression. The sum total of all Samskaras is known as “Karmasaya” or receptacle of works. This is called Sanchita Karma (accumulated works). When a man leaves the physical body, he carries with him his astral body of 17 Tattvas and the Karmasaya as well, to the mental plane. This Karmasaya is burnt by highest knowledge obtained through Asamprajnata Samadhi." - Swami Sivananda