The beard is the hair that grows on the cheeks and chin.
Ibn Manzoor said quoting from Ibn Sayyidih:
Lihyah (beard) includes the hair that grows on the cheeks and the chin.
Lisaan al-‘Arab, 15/243
The hair that grows on the bones that are level with the channel of the ear – the cheekbones – is part of the beard and it is not permissible to pluck it or shave it.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked about what is meant by the beard. He replied:
The extent of the beard is from the cheekbones, level with the channel of the ears, until the bottom of the face. It includes the hair that grows on the cheeks.
It says in al-Qaamoos al-Muheet (p. 387): Lihyah (beard) is the hair of the cheeks and chin.
Based on this, the one who says that the hair that grows on the cheeks is not part of the beard must bring proof of that.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 11/question no. 49
He (may Allaah have mercy on him) was also asked: Are the cheeks part of the beard?
He replied: Yes, the cheeks are part of the beard, because this is what is implied by the language in which sharee’ah came. Allaah says (interpretation of the meaning):
“Verily, We have sent it down as an Arabic Qur’aan in order that you may understand”
“He it is Who sent among the unlettered ones a Messenger (Muhammad) from among themselves, reciting to them His Verses, purifying them (from the filth of disbelief and polytheism), and teaching them the Book (this Qur’aan, Islamic laws and Islamic jurisprudence) and Al‑Hikmah (As‑Sunnah)”
Hence it is known that the meanings brought in the Qur’aan and Sunnah are the meanings indicated by the Arabic language, but if there is a particular shar’i meaning it should be interpreted in that manner. For example, salaah in Arabic means supplication (as in du’aa’), but in shar’i terminology it refers to the specific, well known act of worship (prayer), so when it is mentioned in the Qur’aan and Sunnah, it is to be understood in the shar’i sense, unless there is any indication to the contrary.
Based on this, Islam does not ascribe any particular shar’i meaning to the word lihyah (beard), so it is to be understood in its linguistic sense, and in Arabic it refers to the hair that grows on the cheeks and chin, from the cheekbone that is level with the channel of the ears to the jawbone.
It says in al-Qamoos: Lihyah (beard) is the hair of the cheeks and chin. Similarly it also says in Fath al-Baari, p. 35; vol. 10, al-Salafiyyah edition: It is a name for that which grows on the cheeks and chin.
Thus it is clear that the cheeks are part of the beard, and the believer must be patient in obeying Allaah and His Messenger (peace and blessings of Allaah be upon him), even if it is strange among his own race or people. Glad tidings to the strangers.
It should be noted that truth is weighed according to the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him); it is not weighed according to what people do that goes against the Qur’aan and Sunnah. We ask Allaah to make us and our Muslim brothers steadfast in adhering to the truth.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 11/question no 50.
As for the hair that grows on the neck, it is not part of the beard. Imam Ahmad (may Allaah have mercy on him) stated that there is nothing wrong with removing hair that is beneath the pharynx, as it says in al-Insaaf (1/250), because it is not part of the beard.
Shaykh Muhammad al-Safareeni said:
The reliable point of view, as it says in al-Iqnaa’ and elsewhere, is that it is not makrooh to remove hair that is beneath the pharynx.
Ghidha’ al-Albaab Sharh Manzoomat al-Adaab, 1/433
See also question no. 9037
And Allaah knows best.