Haram is an Arabic term meaning ‘forbidden’. It refers to anything that is forbidden by the Qur’an, Hadith, and Islamic law.
It is the opposite of halal (allowed or permissible). If one does or consumes something haram, they are likely to be punished in the afterlife.
There are many reasons why something might be considered haram. In general, actions that are haram can result in some kind of harm for oneself or others. There are different opinions about what is considered haram.
Text adapted from English Wikipedia, Creative Commons Attribution-ShareAlike License 4.0.
Image from Halal Navi Blog, below threshold for copyright protection.