Transparent
Transparent printing inks that are not opaque are used in four-colour printing.
Special colours such as PANTONE are also usually transparent.
Lexicon
Trimbox
The Trimbox is the final format of a PDF file without bleed. For an A4, the Trimbox of the PDF is therefore 210x297mm, while the Bleedbox would be 216x303mm with 3mm bleed all around.