On "old" doors with keyholes, the escutcheon is the largely decorative piece of metal around the hole serving multiple purposes:
- decoration
- protection of the wood, so it doesn't get worn by multiple key entry,
- guide for aligning the key straight
-reflective material making it easier to find the lock in dim light.

In your case, it will be a rounded / cupped / dished piece of metal around the protruding lock cylinder so that attempts to whack the cylinder with a hammer will be largely defeated, the escutcheon diverting the blow.

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Note added at 3 hrs (2008-01-19 16:09:40 GMT)
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Fitted from 1966 with the door safety escutcheon boss also fitted.This boss is available as 24A1834 and gasket 24A1835 ...
www.minispares.com/Product.aspx?pid=42145

exterior safety escutcheon. -interior safety escutcheon. -screws. •. cable: EL648 – 8853, EL650 – 8855. •. loose magnetic contact: 8951/8952 ...
www.vingcardmarine.com/binary?id

50 Ø x H mm safety escutcheon. for profile cylinder, 50 Ø x 2 mm .... with safety escutcheon and bush at the. top for handle. Threaded lugs ...
www.irsecurityandsafety.co.uk/downloads/irah_randi.pdf

Silly me! SECURITY escutcheon would be much better.
Chubb Security Escutcheon - Souber RR1 Repair Rose ...
www.locksonline.co.uk/acatalog/copy_of_Accessories.html

And as this site says, a security escutcheon also prevents people drilling into the lock (quite so quickly) :
http://www.bosisystems.com/Images/TesaDoor_Web_a.JPG


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Note added at 3 hrs (2008-01-19 16:10:49 GMT)
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Your second term should be self-explanatory now.