The most persistent external description of Sikhism is that Nanak synthesised Hinduism and Islam, taking the best of each. Sikhs reject this consistently, and the rejection is well founded.
The claim usually rests on surface features: a formless God, opposition to caste and to idol worship, use of vocabulary from both Sanskritic and Persian sources. But Nanak's hymns are not eclectic. They set out a position and criticise both traditions from it, and the criticism is directed at ritual, at caste, at renunciation and at religious professionals of every kind. He rejects the sacred thread, the Muslim and Hindu ritual calendars, pilgrimage as merit, and asceticism. He also does something neither tradition does: he insists that liberation is available within ordinary life, in a household, doing honest work.
The Sant tradition of north India, and figures such as Kabir and Ravidas, whose compositions are also included in the Sikh scripture, form the milieu he belongs to. Situating him there is historically defensible and is quite different from calling him a synthesiser.
The institutional distinctness came later and deliberately. The line of ten Gurus ends with Gobind Singh, who in 1699 founded the Khalsa with its initiation and its five articles, and who before his death in 1708 vested authority in the scripture itself as the Guru Granth Sahib rather than in a living successor.
Indian constitutional law has at various points classified Sikhs under a broader Hindu heading for particular statutory purposes, and this has been a live grievance.