Bhai Nand Lal Ji, poet laureate of Sahib Sri Guru Gobind Singh Ji's royal court, writes the following about what the Khalsa really is in "Tankah Nama"

Khalsa so-i jo nindaa ti-aagay
Khalsa so-i laray jo agay
Khalsa so-i jo panchaa ka maaray
Khalsa so-i karna ko saraay

Khalsa is the one who does not slander others
Khalsa is the one who fights in the front line
Khalsa is the one who conquors the 5 evils (lust, greed, anger, excessive attachment, excessive pride)
Khalsa is the one who fulfills all duties

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Khalsa so-i maan jo ti-aagay
Khalsa so-i jo par istaree-aa ti-aagay
Khalsa so-i par drisht ko ti-aagay
Khalsa so-i naam rat laagay

Khalsa is the one who renounces self-pride
Khalsa is the one who remains faithful within marriage
Khalsa is the one who abandons sexual desire for others than the spouse
Khalsa is the one who is blessed with God's Name

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Khalsa so-i guru hit laavay
Khalsa so-i saar mukh khaavay
Khalsa so-i nirdhaan ko paalay
Khalsa so-i dusht kao gaalai

Khalsa is the one who loves Waheguru
Khalsa is the one who fights bravely in battle
Khalsa is the one who helps the needy
Khalsa is the one who overpowers the enemy

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Khalsa so-i naam jap kaaray
Khalsa so-i malaych paar charay
Khalsa so-i naam si-o joray
Khalsa so-i bandan ko toray

Khalsa is the one who chants God's Name
Khalsa is the one who rises above the evil ones
Khalsa is the one who is in tune with God's Name
Khalsa is the one who breaks false rituals

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Khalsa so-i jo charai tarang
Khalsa so-i jo karay nit jang
Khalsa so-i shaastar ko dhaaray
Khalsa so-i dushat kao maaray.

Khalsa is the one who becomes a crusader
Khalsa is the one who fights the war daily against internal and external enemies
Khalsa is the one who is always ready with weapons
Khalsa is the one who destroys all evil.