Mark Fortenberry wrote:
A single page of a publication containing a specific ad or article in print.

www.nahc.org/NAHC/VendorSvcs/Advertising/terms.html

A copy of a published page (magazine or newspaper) in which a model's picture appears. Tearsheets are generally included in a model's portfolio as evidence of work the model has done.

photographytips.com/page.cfm/2093

An actual copy of an advertisement.

retailindustry.about.com/library/terms/blterm_t.htm

Sample of an author's published work; the actual page containing the article, story, etc.

www.thescriptorium.net/glossary.html

Previously -printed material submitted as manuscript (or as art for research purposes); typically an ad or photo torn from a magazine or newspaper.

phbusiness.prenhall.com/authorguide/glossary.html

previously printed book page, usually edited, being used for manuscript.

www.gtscompanies.com/printp_z.html

100% correct!!
This should be included with any good model reference book also...

Rich

MM Jen wrote:
Where's the ruling?

This is the Internet.  There is no ruling.

The references suggested above are very good if you want to conform to what serious industry professionals will think of as a tear sheet.  However, as you already know, lots of people claim other things.

The right question on the 'net, it seems to me, is not "what is a tearsheet", but rather "what kind of publication will be perceived as prestigious" as a real tearsheet might be.  People can claim anything they want, and do - but most of the time those claims act against them.  People seeing that kind of self-published or meaningless published stuff listed as a credit immediately assign the model to the "clueless" category, and further decide that her sense of self-importance is out of calibration.