Though customarily a watermark is within the body of the paper, in the context of your printing it would be clear that this is an applied ink watermark. I think "watermark" is the term MS Word uses (or used) for its facility.

Watermark Traditionally formed by sewing a maker's mark into the mould's wire screen and thusly imparting it to the finished paper, watermarks today are produced in many ways. They can be done by etching a design onto one of the rollers used in the manufacturing process or by ***altering the ink coverage when making a print***.
[http://www.atelieraskart.com/DigitalGlossary01.htm]

Watermarks can be used on documents to provide an effective form of protection against counterfeiting. The watermark images are invisible or difficult to view unless held up to the light or held at a 45° angle. They can be applied as a true watermark which is applied to the paper during the manufacturing process or as an ***artificial watermark which is applied by a printing process***. Either type of watermark provides a security feature that is difficult, if not impossible, to copy by copiers and scanners. Shown below is a description of a true watermark and an artificial watermarks …

Artificial watermarks are ***applied after the paper manufacturing process***. They can be applied by the paper manufacturer or ***by the printer***. Many paper manufacturers offer stock security papers that have a standard artificial watermark applied to them. The cost of the security paper with an artificial watermark is generally a little less expensive than one with a true watermark.
An artificial watermark can be seen from one side only. It is generally applied to the back side but can be applied to the front side also. The watermark is achieved by printing the image in opaque white ink, transparent ink or by using varnish. They will all produce an image that is visible when viewed at an angle. An artificial watermark is sometimes also referred to as ghost printing. Artificial watermarks are nearly impossible to duplicate by copier or scanners, making this a good security feature to add to your documents.
Adding a customized artificial watermark to your document will make it even more difficult to duplicate. Customizing an artificial watermark is less expensive than customizing a true watermark. The customizing can be accomplished by the paper manufacturer or the printer. The most cost effective alternative will depend on the quantity of watermarked paper needed. Consult your supplier as to what would be the best alternative for your needs.
[http://glossary.ippaper.com/default.asp?req=knowledge/articl...]




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Note added at 53 mins (2005-02-22 10:53:17 GMT)
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Indeed :

Microsoft Word Technology Tips. Creating a Printed Watermark (May 19, 2004). ... They use a special feature called a Printed Watermark . ...
scnc.misd.k12.mi.us/techtips/word/watermark.html - 10k - Cached - Similar pages


Teach your Word 2000 users to master watermarks... The Printed Watermark dialog box lets you specify the text, font, size, color, and horizontal or diagonal positioning or change any of the options. ...
techrepublic.com.com/5100-6270-5035237-2.html - 42k - Cached - Similar pages


Word 2003: Adding Watermarks, UWEC... From the Format menu, select Background » Printed Watermark... The Printed Watermark dialog box appears. ... The Printed Watermark dialog box appears. ...
www.uwec.edu/help/Word03/watermark.htm



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Note added at 57 mins (2005-02-22 10:57:11 GMT)
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Indeed :

Creating a Printed Watermark in WordMicrosoft Word Technology Tips. Creating a Printed Watermark (May 19, 2004). ... They use a special feature called a Printed Watermark . ...
scnc.misd.k12.mi.us/techtips/word/watermark.html - 10k - Cached - Similar pages


Teach your Word 2000 users to master watermarks... The Printed Watermark dialog box lets you specify the text, font, size, color, and horizontal or diagonal positioning or change any of the options. ...
techrepublic.com.com/5100-6270-5035237-2.html - 42k - Cached - Similar pages


Word 2003: Adding Watermarks, UWEC... From the Format menu, select Background » Printed Watermark... The Printed Watermark dialog box appears. ... The Printed Watermark dialog box appears. ...
www.uwec.edu/help/Word03/watermark.htm - 9k

Have just checked the French Word, where Format - Arrière-Plan - Filigrane imprimée gets you to this feature.