The term חֲתִימַת זָקָן — literally the "sealing" or full growth of the beard — appears explicitly in Bava Metzia 39b, where the Gemara explains the pasuk "and Joseph knew his brothers but they knew him not" (Bereishit 42:8, as cited in Bava Metzia 39b) to mean that Joseph had left Eretz Yisrael without חֲתִימַת זָקָן and returned with חֲתִימַת זָקָן, demonstrating that a full grown beard so transforms a person's appearance that even brothers may fail to recognize him.
The Torah prohibition that gives the beard its legal significance is stated in Vayikra 21:5, which forbids shaving the corners of the beard, and this prohibition is mapped onto five distinct edges by Mishneh Torah, Foreign Worship 12:7, which identifies them as the upper and lower jaw on each side together with the chin — the very anatomy that חֲתִימַת זָקָן, when complete, covers in full.
The Mishnah in Makkot 20a enumerates those five edges of the beard and specifies that liability requires removal with a razor, a principle derived through a gezeirah shavah in Sifra, Kedoshim 6:4 and Nazir 58b: scissors alone do not constitute השחתה (destruction), while tweezers alone do not constitute גילוח (shaving) — only a razor combines both, making it the sole biblically prohibited instrument.
Makkot 21a records that Rav Sheshet would gesture physically at the junctures between the sections of the beard when teaching which points mark the five edges, underscoring that חֲתִימַת זָקָן describes the beard as it frames and fills precisely those anatomical divisions.
Shulchan Arukh, Yoreh De'ah 181:10 rules that liability applies only when a razor is used, while scissors are permitted even when cutting close, and the Rema adds that a God-fearing man should not pass a razor over any part of his beard at all — a ruling that presupposes the full, sealed beard of חֲתִימַת זָקָן as the baseline state being protected.