Fear is the sensation that results from the anticipation of an expected harm; it is also felt when a person loses, or expects to lose something he cherishes. It is the exact opposite of security and can be applied to worldly matters as well as those of the Hereafter. Fear is a state of disturbance of the heart from an evil or harm that may befall a person, or, it can be the sensation it feels when missing out on something that is beloved to it.

Imam Ibn Qudamah said,

“Fear is no more than the pain and burning that one finds in his heart due to an expected harm; it is like the example of one who committed a crime and was subsequently caught; such a person will fear being sentenced to death, and the pain which his heart will suffer is dependent on how certain he is that he will (or will not) be sentenced, depending on the severity of his crime … sometimes one fears the ruler due to the power of the latter. Thus, when one realises the power of Allah and knows that if Allah wished to destroy all His creation it would not affect Him, nor could anyone stop Him from doing so, and therefore, one would fear Allah to the extent of the sins that he has committed. This is the fear of warning.”

There is a certain type of fear that is experienced only by the knowledgeable, and it is called Khashyah in Arabic, as Allah The Almighty Says (which means):

{“…Only those among His servants fear Allah who have knowledge…”} [Quran, 35: 28]

This is fear that is based on knowledge. Ordinary believers experience a general fear of Allah The Almighty whilst Khashyah is only experienced by the devout believers and the scholars from among them; the more knowledge one has regarding Allah The Almighty, the more he will fear Him. When one fears a person who may or may not apprehend him then this is an ordinary fear, whereas if he is absolutely sure that this person can seize him and inflict harm upon him, then it is Khashyah.

Fear, with its various different meanings, is mentioned in many places in the Quran, and its causes are varied:
• Fear of being killed or defeated; Allah The Almighty Says (what means):

{“And when there comes to them something [i.e., information] about [public] security or fear…”} [Quran, 4:83]
• Fear during war and hostility; Allah The Almighty Says (what means):
{“… And when fear comes, you see them looking at you, their eyes revolving like one being overcome by death. But when fear departs, they lash you with sharp tongues…”} [Quran, 33: 19] 'Fear' in this verse refers to war.
• Fear resulting from knowledge, as Allah The Almighty Says (which means):
{“But if one fears from the bequeather [some] error…”} [Quran, 2 :182]
Allah The Almighty also Says (what means): {“…Unless both fear that they will not be able to keep [within] the limits of Allah…”} [Quran, 2: 229] 'Fear' in this verse is due to one's knowledge.
• Fear due to decrease in numbers, as Allah The Almighty Says (what means): {“Or that He would not seize them gradually [in a state of dread].”} [Quran, 16: 47] This is the fear that people feel regarding a decrease in their numbers, which would be the result of them being killed, or facing death in any other way.
• Fear of punishment, as Allah The Almighty Says (what means):
{“…They [i.e., the believers] supplicate their Lord in fear…”} [Quran, 32: 16]

Imam Ibn Qudamah said, “Know that fear is the whip by which Allah controls His slaves so that they may continue exerting effort in order to come close to their Lord; fear is the light which illuminates the heart and enables it to distinguish good from evil.” When a person fears another, he will attempt to flee from him, and this is the case for everyone except Allah; one escapes from Him by fleeing to Him.

Fear does not depart from one's heart except that it causes its ruin, because when fear is present in one's heart, it burns all desires and expels the pleasures of this life from it. Fear causes a person to wake up from his heedlessness and puts him back on the right path after he has diverted from it. Being fearful is not an objective in itself, meaning, we do not feel fear due to any contentment that comes with being afraid, but rather, it should be a means to rectify our condition.