How do you define ghosts? Souls of dead people who have some unfinished business or had violent deaths etc?

Muslim belief says that after a person dies, his soul is taken and placed in a state called Barzakh. Which is something like a staging area before the day of judgement (Sorry I am learning data management). No soul can escape from that state.

Muslims generally believe in three (or two, depending on how you look at it) kinds of supernatural entities.

<ol style="margin: 0px 2em 1em 0px; padding: 0px; list-style-position: initial; list-style-image: initial; outline: 0px; color: rgb(51, 51, 51); font-family: q_serif, Georgia, Times, "Times New Roman", "Hiragino Kaku Gothic Pro", Meiryo, serif; font-size: 14px; background-color: rgb(255, 255, 255);"><li style="margin: 5px 0px 5px 2em; padding: 0px 0px 0.7em; outline: 0px; position: relative;">Angels - Created by Allah to worship Him and obey His commands. They do not have free will so there is no concept of fallen angels in Islam. They cannot fall because they cannot sin or revolt against Allah.</li><li style="margin: 5px 0px 5px 2em; padding: 0px 0px 0.7em; outline: 0px; position: relative;">Iblis and his minions: Iblis or Satan is the one who misguides people, creates fights between couples, whispers into hearts to create confusion and mischief etc. He also has his minions to do his work.</li><li style="margin: 5px 0px 5px 2em; padding: 0px; outline: 0px; position: relative;">Jinn: Like humans, they have free will and can be either good or evil. They also live as communities, forming families etc. They are made of fire. The Quran has a Surah which talks about a group of Jinn who heard the Quran and become Muslims after that. <a href="https://quran.com/72" mobile_app_target="browser" rel="noopener nofollow" target="_blank" class="external_link" style="margin: 0px; padding: 0px 15px 0px 0px; -webkit-tap-highlight-color: rgba(34, 106, 173, 0.1); user-select: none; color: rgb(43, 109, 173); text-decoration-line: none; outline: 0px; background-image: url("//qsf.ec.quoracdn.net/-3-images.new_grid.external_link.svg-26-aef78ead48f1f1e2.svg"); background-position: right 0.3em; background-size: 10.5px; background-repeat: no-repeat; background-attachment: initial; background-origin: initial; background-clip: initial;">Surah Al-Jinn [72]</a></li></ol>

In the end, Allah knows best. I hope this helps.

People are afraid of ghosts, because people are afraid of their sins

Yes. Ghosts most likely DO exist everywhere. And every culture interprets them differently.

I believe but actually this is very personal and depend of the reginal cultural.

Islam believes in the unseen. We don't believe in ghosts.

Let me specify. What is a "ghost?" Typically, this means "the soul or spirit of a dead person who haunts the world of the living." This definition contradicts Islam; once a soul leaves, it never returns until the Day of Resurrection.

There are many ayaat and ahadith that talk about what happens to a human soul when it dies. To summarize:

<ul style="box-sizing: inherit; margin: 0px 0px 0.8em 2em; padding: 0px; border: 0px; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 15px; vertical-align: baseline; list-style-position: initial; list-style-image: initial; color: rgb(12, 13, 14); background-color: rgb(255, 255, 255);"><li style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font: inherit; vertical-align: baseline;">The angel of death comes and extracts the soul</li><li style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font: inherit; vertical-align: baseline;">The angels wrap it and carry it</li><li style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font: inherit; vertical-align: baseline;">It is eventually returned to its grave</li><li style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font: inherit; vertical-align: baseline;">The person is asked the questions of the grave</li><li style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font: inherit; vertical-align: baseline;">They remain there until resurrection day</li></ul>

Going back to ghosts: on the other hand, we have Jinns, which are mentioned in the Qur'an: invisible beings of great physical strength. They can possess (enter into and speak on behalf of) humans.

Among the proofs of this are:

<blockquote style="box-sizing: inherit; margin: 0px 0px 0.8em; padding: 10px; border-width: 0px 0px 0px 2px; border-top-style: initial; border-right-style: initial; border-bottom-style: initial; border-left-style: solid; border-top-color: initial; border-right-color: initial; border-bottom-color: initial; border-left-color: rgb(255, 235, 142); border-image: initial; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 15px; vertical-align: baseline; quotes: none; background-color: rgb(255, 248, 220); color: rgb(12, 13, 14);">

Those who consume interest cannot stand [on the Day of Resurrection] except as one stands who is being beaten by Satan into insanity <em style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font-variant: inherit; font-weight: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; font-size: inherit; vertical-align: baseline;">being possessed</em>. That is because they say, "Trade is [just] like interest." But Allah has permitted trade and has forbidden interest. <a href="http://quran.com/2/275" style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant: inherit; font-weight: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; color: rgb(0, 119, 204); text-decoration-line: none; cursor: pointer;">Surah Baqarah, verse 275</a>

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