Sikhism is regularly grouped with Hinduism, Buddhism and Jainism as a Dharmic or Indian religion, on the grounds of shared geography, a shared vocabulary of karma, rebirth and liberation, and a common setting. Many Sikhs accept the grouping as a matter of description. Others object to it, and the objection is worth stating plainly, because it bears on how the tradition understands itself.
The Sikh account is that Guru Nanak received a distinct revelation rather than reforming something already there. On that view the shared vocabulary is the language of the time and place rather than evidence of derivation, and what the Gurus taught (one formless God, never incarnate, no image, no caste, no priesthood, a scripture that is itself the Guru), departs from the traditions around it at precisely the points that matter.
The description most firmly rejected is that Sikhism is a syncretism, a fusion of Hindu and Muslim elements. It was a common framing in colonial-era scholarship and it still appears in general accounts. Most Sikhs reject it, and the scholarly case against it is that it mistakes a shared idiom for a shared source and cannot account for the specific positions the Gurus took against both traditions. Placing the Gurus' teaching in relation to the wider devotional currents of northern India is a different and considerably less contested claim than fusion, and many historians do that.
The question has had legal weight. Indian law has at times treated Sikhs as falling within a Hindu category for the purposes of personal law, which Sikh bodies have contested, and separate recognition has been a persistent demand.