For what it's worth: From the PNAS website, what an "Embargo" means:
PNAS Embargo Policy
http://www.pnas.org/site/misc/journalist.shtml#embargo
Purpose | Eligibility | Conditions
Purpose
Every week, PNAS sends out a "Tipsheet" alerting eligible members of the media to forthcoming articles that might be of interest. PNAS makes the full text of all articles available in advance of publication. Distribution and use of advance materials from PNAS is controlled by the embargo policy. This policy is designed to provide news reporters an opportunity to write accurate news stories while ensuring that publicity does not appear prematurely. Through consistent implementation of our embargo policy, we work to maintain a fair and level playing field that gives no one reporter or organization an advantage over others.
Eligibility
Staff journalists or freelancers writing for daily or weekly publication(s) and public information officers from research institutions are eligible to receive advance, embargoed material from PNAS. Writers of internal newsletters and suppliers of financial advice are not eligible. Eligible persons must agree to abide by the conditions of the PNAS embargo policy.
Conditions
■Precise embargo dates accompany all advanced materials (typically 3 PM Eastern time the Monday of online publication). Embargoed information is not to be made public in any format, including print, television, radio, or via the Internet, before the embargo date.
■Advanced material is for background research only. Journalists may not redistribute the information within or outside their news organization, with the exception that journalists may share embargoed papers with experts in the field for the purpose of obtaining relevant commentary. Embargoed materials must be clearly labeled with the embargo date and time. Journalists are responsible for ensuring that all third parties honor the embargo date.
■PIOs may not post, release, or distribute embargoed material in any publicly accessible manner before the embargo date. However, PIOs may distribute embargoed material directly to the media the Wednesday before the embargo date. Distributed material must be clearly labeled with the embargo date and time. PIOs are responsible for ensuring that all third parties honor the embargo date.
■Journalists and PIOs should credit "Proceedings of the National Academy of Sciences" as the source of the information. Please note that the articles in PNAS report original research by independent authors and do not necessarily represent the views of the National Academy of Sciences or the National Research Council.
■Supporting materials provided by PNAS, such as graphics, audio, and video clips must be credited to the appropriate source. These materials may not be used to illustrate stories unrelated to the relevant PNAS article without express permission from PNAS.
■Failure to adhere to this policy will result in revocation of access to all embargoed material from PNAS and the National Academy of Sciences.