الضَّلَالُ: الْعُدُولُ عَنِ الطَّرِيقِ الْمُسْتَقِيمِ، وَهُوَ ضِدُّ الْهِدَايَةِ
Turning away from the Straight Path and it is the opposite of guidance.
(Taken from Aqeedatut Tawheed of Shaykh Saalih al Fawzaan)
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Islam: The Definitions 