Folio
The page number. A 'blind folio' is a counted page that doesn't show its number (common on chapter openers).
More in Design & type
Bleed
Artwork extended past the trim line (usually 3 mm / 0.125") so that, after cutting, colour or images reach the very edge with no white slivers.
Drop cap
An enlarged opening capital letter that drops into the first lines of a chapter or section — a classic typographic flourish.
Gutter
The inner margin near the spine, plus the allowance for the binding. Too little gutter and text disappears into the fold.
Justification
Setting text so both left and right edges align. The opposite, 'ragged right', aligns only the left edge.
Kerning
Adjusting the space between specific letter pairs so spacing looks even — e.g. tightening 'AV' or 'To'.
Leading
The vertical space between lines of type (from 'lead' strips in metal type). Sensible leading is what makes body text comfortable to read.