The Definition of Sikh

Any human being who faithfully believes in
i. One Immortal Being,
ii. Ten Gurus, from Guru Nanak Sahib to Guru Gobind Singh Sahib,
iii. The Guru Granth Sahib,
iv. The utterances and teachings of the ten Gurus and
v. the baptism bequeathed by the tenth Guru, and who does not owe allegiance to any other religion, is a Sikh.

Reference: Sikh Rehat Maryada, Chapter 1, Article 1.