Hermes Trismegistus was known as the author of a canon of works (including astrological and alchemical works) known as the Hermetica that was influential on medieval and renaissance scholars (e.g., being cited as an authority by Copernicus *). The expression of a hermetic seal derives from an alchemical procedure reported in the canon.
Hermes Trismegistus was a legendary figure sometimes considered to be a contemporary of the biblical Moses, or to be the prophet Enoch, and seen as linked to both ancient Greek (i.e., Hermes) and Egyptian gods. Trismegistus (the 'thrice great') was considered a wise spiritual leader, and he was respected in Christian and Islamic traditions.
It was realised in the 17th Century that the Hermetica was not as ancient as had been assumed, and probably comprised of collected writings of a range of different Greek authors. If Hermes Trismegistus was a real historical figure, he was not the author of the works named for him.
(* sun is a lantern which is the lamp of a very beautiful temple)
