The Nirmalas are a Sikh scholarly order. The tradition holds that Guru Gobind Singh Ji sent five Sikhs to Benares to learn Sanskrit, and that they returned and founded the order.
They provided the Sikh tradition's scholarship for two centuries: commentaries on the Guru Granth Sahib Ji, Sanskrit learning, and the interpretation of Sikh doctrine in the vocabulary of Vedantic philosophy.
That last point is why the Singh Sabha reformers opposed them. The Nirmala reading placed Sikhi within a broader Indic frame; the Tat Khalsa position was that it stands separately. The dispute was decided institutionally in favour of the reformers.
Nirmala akharas held property at Haridwar, Allahabad and Punjab, and Nirmala scholars ran the deras where Sikh boys were educated before the Khalsa schools existed.
The Faridkot Teeka, the nineteenth-century Punjabi commentary on the Guru Granth Sahib Ji, came from this tradition. It is still consulted and its framework is one the tradition that produced it lost the argument about.
The name is from nirmal, spotless. Its members wear white or ochre and are celibate, which sets them apart from a householder tradition, and they organise as akharas on the pattern of the Hindu monastic orders and take part in the Kumbh. That last is precisely what the Singh Sabha objected to and it is still what the order is identified by.
Pandit Tara Singh Narotam, a Nirmala, compiled the Sri Gur Tirath Sangrahi in the nineteenth century, the first systematic register of Sikh shrines and the places associated with the Gurus. It is the earliest list of what the tradition then counted as its own, and it is used by people who would never otherwise open a Nirmala book.
The Singh Sabha reform rejected the Vedantic frame and the Nirmala interpretation lost its place. The order continues, with akharas at Patiala, Haridwar and elsewhere, and its literature is largely unread.