According to ulama ushul fiqh, there are two definitions of haram, namely in terms of boundaries and essence, and in terms of form and nature.
In terms of its limits and essence, Imam al-Ghazali formulates haram with what is required of shari'ah (Allah SWT and His Messenger) to be abandoned through demands with certainty and binding. In terms of form and nature, Imam al Baidawi, the figure of ushul fiqh of the Shafi'i School, formulates haram by 'an act whose perpetrators are reproached'. There is also the ushul jurisprudence scholars who add in the formula with the phrase '..and the one who left it is praised', as opposed to mandatory sense.
Similar and unlawful terms in ushul fiqh are al-mahzur (which is avoided), al-ma'siyah (maksiat), az-zanb (sin), al-mamnu (forbidden), al-qabih (the bad / ugly), as-sai'ah (ugly), al-fahisyah (the heinous), al-ism (sin) and al-mazjur'anh (which prevented him).
For the cleric of the Hanafi School, a proposition which refers to the haram's law of quality must be a qat'i (definite) proposition. If the proposition is of zanni (relative) quality, then they are called makruh tahrim. While jumhur ulama ushul fiqh does not distinguish between the qat'i and zanni theorem. According to them, the origin of the proposition refers to the expressions referring to prohibition, both the proposition qat'i and zanni, then the law remains haram.
Meanwhile, the division of haram is twofold. If the prohibition is related to the essence of the act, it is called haram li zatih (haram because the substance). And when it comes to something outside the essence of the forbidden, but in the form of mafsadat-an, it is called haram li gairih (haram for the others).
More specifically, haram li zatih is a prohibition that since the prescribed syar'i semarang that it is haram, for example, eating carcasses, pigs, drinking alcohol, adultery, killing and eating the wealth of orphans. Prohibition on the things above is a prohibition on the substance (essence) of the work itself.
While haram li gairih is something that at first disyariatkan, but accompanied by a mudharat for humans, keharamannya caused by the mudharat. For example, performing prayers with garments that are gasab (take the goods of others without permission), transact buying and selling when the call to prayer prayer Friday, or fasting on the feast of Eid al-Fitr.
There are differences of opinion ulama ushul fiqh in determining the law of haram li zatih's actions, whether void (vanity) or fasid. The Hanafi School of Ulama thinks, in muamalah matters, because the prohibition is not on the substance, but due to external factors, the law is fasid (broken), not void. However, if haram li gairih which concerns aspects of worship, the law is void.