What's the rectangle surrounding a font glyph, including its sidebearings, called? I Googled the words "bounding box" and got conflicting results, where some sources say it's the rectangle that bound the glyph tightly (without sidebearings), and others say it includes sidebearings.
Okay, so after doing quite a bit of research, here's my conclusion: there seems to be no perfect English term. But there is one workable candidate, although it won't be very useful in English: "body". This is a reference to the "body" of a sort in movable typesetting, as suggested by user K Pease on Typedrawers. It is in fact still used in Japanese type design, specifically as 仮想ボディ ("imaginary body"), given that "bodies" are much more important as a concept in East Asian calligraphy and typography than Western ones. "Imaginary body", although a literal translation, is indeed used in some very limited English materials translated from Japanese, so it's not completely terrible. Adobe translates this term as "embox", which can be confused with the concept of the "em square", but given that the height of this box is always exactly one em, it's not a bad one either. The only problems with "imaginary body" and "embox" is that I haven't found any concrete demonstration of these terms on Western text in Japanese sources (most examples are strictly about Japanese text), but I hazard it wouldn't be that different anyway.
In short, "body", "imaginary body" or "embox". Not perfect, but still usable to some extent.