فَالشِّرْكُ الْأَكْبَرُ: مَا يُخْرِجَ الْإِنْسَانَ مِنَ الْمِلَّة. وَالشِّرْكُ الْأَصْغَرُ: مَا دُونَ ذٰلِك

So shirk akbar (greater shirk) is: that which takes a person out of the religion.

And shirk asghar (the lesser shirk) is: what is less than that.

(From Al Qawlul Mufeed of Shaykh al Uthaymeen)

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