[Surah Al-Qalam 68:4]
Arabic definition of Zulm - To remove something from its deserving position and put it elsewhere or place something in a position it does not deserve.
Allah has placed everything in its deserving position and it should not be removed from that position. Shirk, or associating partners with Allah is the biggest form of zulm.
“Indeed Shirk is the biggest form of zulm” (Surah Luqman)
This is the greatest form of Zulm because the person has put something or someone un-deservedly, in the position of God.
You might be committing one of these forms of zulm!
If a person ignores the status of parents and gives utmost respect to someone else, then he is said to have done injustice or zulm.
If a person gives the right of a wife to his girlfriend, then it is a form of zulm, too.
If a miser does not give charity to the poor, or withholds the rightful shares of others from his wealth, then this is also zulm.
Every zulm will be a darkness for the one who commits it - says a Sahih Hadith. No matter how many acts of worship you perform, if you have transgressed against someone’s rights, on the day of Judgement you would be standing amongst Zalimeen (opressors). A person must seek forgiveness from his victims, if he has committed any sort of zulm upon others, no matter how small it appears to be; even if it is a loud shout without any reason.