I just came back after attending a talk by Professor Cynthia Mahmood on Sikhs, and she raised this question in the beginning of her chat: Who is a sikh? And then attempted to answer in many different ways what a Sikh was. She was talking about her interviews with extremists.
The most traditional definition is a Sikh is a disciple of Guru Nanak and the 9 Gurus following him, and the Guru Granth Sahib. But, because the rest is really an essense of Guru Nanak's teaching, it is apt to say that a Sikh is a disciple of Guru Nanak.
Then the question arises, who is Guru Nanak? Is it that person that was born in the 15th century that apparently started "Sikhism"? I hardly know that Guru Nanak. The Guru Nanak I know sings poems now. Guru Nanak is not born and cannot die. Guru Nanak lives as long as there is the singing of the creator. So Guru Nanak is the singer among us. All of us. Me and you, and every thing surrounding us and inside us.
Kabir sings, "Saying 'You! You!' I have become You; Now wherever I see, I see You." There is nothing that does not sing. So, there is nothing that is not Guru Nanak. The disciple of Guru Nanak is one who realizes that the singer within him, and around him is Guru Nanak.