Definition - What does Shuddhi mean?
Shuddhi is a Sanskrit word meaning "purification," "cleansing" and "freedom from defilement." Shuddhi also refers to the early 20th century movement to bring those who had converted to other religions back to Hinduism; in essence, purifying them for return to their religion.
In yoga, bhuta shuddhi is a sadhana (spiritual practice) that purifies or stabilizes the five elements: water, air, earth, fire and space.
Yogapedia explains Shuddhi
Bhuta shuddhi refers to the yoga practice of freeing the practitioner from his/her physical nature to become one with the higher Self, or Brahman. In some types of yoga, it is considered a kriya, which is a series of postures, meditation, breathing exercises and sounds or mantras to reach an outcome (in this case, oneness with the Divine). Practicing bhuta shuddhi creates physical, emotional and mental changes that cleanse the body, mind and soul.
The shuddhi movement was started in the early 20th century by Arya Samaj bringing non-Hindus back to Hinduism and stemming the conversions to Islam and Christianity. It developed from the ancient process of reconverting those who had been forced to convert to other religions. This process was known as shuddhkarin. The shuddhi movement expanded the process to include outcasts from other religions, not just reconversions. There was also a reform agenda targeting the social structure that had created an untouchable class.