प्रशान्तजगदास्थोऽन्तर्वीतशोकभयैषणः ।स्वस्थो भवति येनात्मा स समाधिरिति स्मृतः
In plain words
That by which the self comes to stand in itself -- its reliance on the world grown wholly calm, inwardly free of grief, fear, and desire -- that is what is known as samādhi.