What is the definition of the beard according to sharee’ah?

Shaykh Ibn Baaz (may Allaah have mercy on him) said: 

The beard (al-lahyah) is that which grows on the cheeks and chin, as was explained by the author of al-Qaamoos. It is obligatory to leave the hair that grows on the cheeks and chin, and not to shave it or cut it. 

May Allaah reform the affairs of all the Muslims. 

Fataawa Islamiyyah, 2/325. 

Shaykh Ibn ‘Uthaymeen said: 

The definition of the beard (al-lahyah) as stated by the scholars of the (Arabic) language is the hair of the face, jawbone and cheeks, in the sense that everything that grows on the cheeks, jawbone and chin is part of the beard. Removing any part of it also comes under the heading of sin, because the Messenger (peace and blessings of Allaah be upon him) said: “Leave the beard alone” and “Let the beard grow.” This indicates that it is not permissible to remove any part of it, but the sin varies in degree; shaving is more serious than removing a part of it because it is a more extreme action and the one who shaves it more clearly goes against the command than the one who trims it. 

Fataawa Hammah, p. 36 

See also Question no. 8196.