The
date something was originally
published.
This is important information for Everything (especially for the Everything Copyright Problem), because the length of time a work is protected by U.S. copyright often depends on this; the term is currently 95 years after publication, but see below. The exception is for works created by a single author, and not made for hire for some company; the copyright on these works lasts 70 years after the author's death.
However, the law establishing this copyright term only went into effect on October 27, 1998, a day that will live in infamy, for it essentially established that no more material would enter the public domain for another 20 years.
Before this date, the term was 75 years after publication or 50 years after death for single-author works not made for hire.
As a result, only compiled or hired works published before October 27, 1923 and works produced by single authors who died before October 27, 1948 are in the public domain.
Note that this is why we can have the full text of Webster 1913 here; it became public domain in 1988.
Source: the text of the Sonny Bono Copyright Term Extension Act at http://thomas.loc.gov/cgi-bin/bdquery/z?d105:SN00505:@@@L|TOM:/bss/d105query.html|