TT #9 · Parashas Noach · First Aliyah · Passuk 6:14
Make for yourself an ark of gopher wood; you shall make the ark with compartments, and you shall cover it inside and outside with pitch.
Torah Temimah points to the identification of “atzei-gopher” (Sanhedrin 108b / school of R’ Shila).
E-Daf image — Sanhedrin 108b (size=3)
Shas / HebrewBooks PDF of the daf
English: “Gopher wood” — What is gopher? The school of R’ Shila said: This is a kind of cedar (or a resinous wood called mabligah / gulmish).
Custom (lexical identification of a rare biblical term).
“Atzei-gopher” refers to a particular species of strong, resinous wood (commonly identified with a type of cedar or similar).