Is what I think.

The following suggests this might be what it is:
Film
Using film to burn plates is still preferred throughout the printing industry. However, the potential problems affiliated with this process are numerous. Film with insufficient density can cause dot gain, dark highlight areas, and plugged-up shadow areas. On the other hand, film that is too dense can result in washed out images (Blatner & Roth, 1993). Other potential problems related to film include scratches, dimensional instability, drift factors during plate exposure, and variations caused by alignment and stripping by hand (Maas, 1995). Direct-to-plate technology tackles these obstacles by "eliminating film, such as color separations, duplicate film and final film" to enhance image quality (Fenton, 1998). With film gone, however, the role of the printing plate becomes very important.

source: http://www.uh.edu/~jwaite/DTOPLATE.html

And then I found the proof:
Burn - To expose a proof or plate to light.
Double Burn - To expose a printing plate to two negatives, creating a composite image
source: http://www.unt.edu/printingservices/about/terms.htm

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Note added at 2003-09-18 12:14:57 (GMT)
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And here\'s another helpful glossary:

http://www.supportsys.net/glossarypgs/list_b.htm

Burn: To expose light sensitive media to light (i.e., burning a negative, burning a printing plate or burning a CD).