Shuddhi is Sanskrit for purification. In Hinduism it is a part of worship. It also refers to converting to Hinduism.
Shuddhi movement
The socio-political movement, derived from ancient rite of shuddhikaran, or purification was started by the Arya Samaj, and its founder Swami Dayanand Saraswati and his followers like Swami Shraddhanand, who also worked on the Sangathan consolidation aspect of Hinduism, in North India, especially Punjab in early 1900s, though it gradually spread across India. Shuddhi had a social reform agenda behind its belligerent rationale and was aimed at abolishing the practise of untouchability by converting outcasts from other religions to Hinduism and integrating them into the mainstream community by elevating their position, and instilling self-confidence and self-determination in them. The movement strove to reduce the conversions of Hindus to Islam and Christianity, which were underway at the time.
In 1923, Swami Shraddhanand founded the 'Bhartiya Hindu Shuddhi Mahasabha' (Indian Hindu Purification Council) and pushed the agenda of reconversion peacefully, but ultimately created a flashpoint between Hindus and Muslims as it offended Muslim exceptionalists, who argued that Hindus, being dhimmis, do not have any rights to convert others to their faith unlike the Muslims, who are mumins. The main point of contention was the reconversion Malkana Rajputs in western United Province Subsequently the movement became controversial and antagonized the Muslims populace to no end and also led to the death of the leader of the movement, Swami Shraddhanand in 1926. Post Swami Shraddhanand death this movement continued with similar missionary zeal.
On 23 February 1928, many Catholic Gaudes in Goa were re-converted to Hinduism notwithstanding the opposition of the Church and the Portuguese government. This was carried out by a Hindu religious institution from Mumbai known as Masurashram, the converts were given Hindu names, but the Portuguese government put impediments in their way to get legal sanction for their new Hindu names. 4851 Catholic Gaudes from Tiswadi, 2174 from Ponda, 250 from Bicholim and 329 from Sattari were re-converted to Hinduism after nearly 400 years. The total number of the converts to Hinduism was 7815. However, in Northern India this movement faced stiff opposition from Sunni Barelvi organisation All India Jamaat Raza-e-Mustafa in Bareilly city which initiated counter to Shuddhi Movement to save Muslims from being converted into Hinduism in British India.