In Islamic terminology, "Halal" and "Haram" are foundational concepts that define the boundaries of permissible and impermissible actions and things for a Muslim.

Shaykh Ibn 'Uthaymeen explained that the "Halal" (حلال) is that which Allah and His Messenger have permitted. The basic principle in Islam regarding worldly matters and customs (al-'adat) is that everything is permissible by default unless there is specific, authentic evidence (daleel) from the Qur'an or Sunnah to prohibit it. This is known as the principle of al-asl fi'l-ashya' al-ibahah (the default ruling on things is permissibility). Therefore, a Muslim is not required to find evidence to prove something is Halal; rather, the burden of proof is on the one who claims something is Haram.

On the other hand, "Haram" (حرام) is that which Allah and His Messenger have explicitly forbidden. Committing a Haram act is a sin for which a person is held accountable and may be subject to punishment in the Hereafter. Conversely, a Muslim is rewarded for abstaining from Haram acts out of obedience to Allah. Declaring something Haram is a serious matter that cannot be done based on personal opinion, cultural norms, or weak evidence. Allah has condemned those who prohibit what He has made lawful or vice versa. The authority to declare something Halal or Haram belongs to Allah alone, conveyed through His revelation.

An important distinction is that for acts of worship (al-'ibadat), the default principle is the opposite: everything is considered an impermissible innovation (bid'ah) unless there is specific evidence from the Qur'an and Sunnah to prove it is a prescribed act of worship.