Arabic, meaning unlawful or forbidden; sometimes considered synonymous with sin. In Islam, proscribed by Allah; unclean. Such things include consumption of alcohol, extramarital sex and eating unacceptable food. The contrary of Haram is Halal (which, I may add, should not be compared to the Jewish kosher which is much more limited in scope) and an intermediate grey area which is not specifically addressed in the Qu'ran is known as Mushbooh (doubtful or suspect).
In the first line of haram are the following expressly forbidden items:
- Pigs/pork and by-products.
- Blood and blood by-products.
- Carnivorous animals.
- Reptiles and insects.
- Halal animals which are not Zabiyah (slaughtered according to Islamic law).
- The carcasses of animals dead before slaughtering (carrion).
- Alcohol and other intoxicants.
Any activity that's forbidden is also considered haram. The concept of haram includes things such as abusing one's wife or squandering household money--things which other cultures regard as merely distasteful or stupid rather than forbidden.
Muslims violating these rules must perform Kaffarah (penance) which often involves charity, one of the five pillars of Islam. The ultimate penalty for unrepentant offenders, as with all Semitic religions is, of course, hell.