Design Nomenclature / Type
Subsetting
Stripping a font file to the glyphs and tables a document actually needs. A Latin subset of a pan-European face can shed most of its bytes. Unicode-range splits a family into subsets the browser fetches only when the characters appear, which is how CJK webfonts become deliverable at all.
In practice
Over-subsetting is the failure mode: strip too far and a name, a currency symbol, or a user-generated comment falls back mid-sentence. Subset to what the site can contain, not to what today's copy contains.