<unk> s are<unk> s in which the free availability of the content is available, but only after several months, with the immediate availability being limited to subscribers.  By its nature, it applies only to electronic versions of<unk> s. 
This variation on<unk><unk>  does not meet the standards for a true<unk><unk><unk> , and such<unk> s are not included in the lists of<unk><unk><unk> s, such as DOAJ. Nonetheless, it does provide useful<unk>  for many purposes.  The term<unk> <unk><unk> " can, in principle, be used for other material as well.
The purpose of this delay is to permit<unk>  to the general public, while still providing incentive to researchers -- and especially research libraries -- to continue their subscriptions. The assumptions behind this model are:
researchers will want the most recent papers in their subject immediately after publication, and will therefore pay the necessary subscription cost, generally several hundred or several thousand dollars.
students and others in smaller non-research institutions do not generally need to see the article as quickly, for they will be using it for a term paper or similar project, not for true research.
the libraries serving such colleges will usually have discontinued such<unk> s (or never subscribed to them) because of their cost. The publisher will thus lose no revenue.
the marginal cost of distributing an electronic<unk>  to additional users is very small; almost all of the expense is preparing the publication and its electronic version in the first place, and the publishers will therefore spend little additional money, while familiarizing undergraduates with their publications.
The difficulties with this model are
it does not serve all academic readers. Those at small colleges also want to keep up with the current literature. In particular those there should be encouraged to be researchers in spite of the difficulties for not being in a major centre, and they would be further harmed by<unk><unk>  to the literature
it will not serve key segments of the general public. The public readers of biomedical<unk> s in particular are typically people concerned about particular diseases that they or their loved ones have. This segment of the public is interested in the current findings, for they will hear news of them.
in some subjects the delay necessary to protect publishers would be very long, especially in some of the "softer" subjects.
Delayed open access journal
Hybrid open access journal
Public domain
Clinical trials terminology
Prevention template
Bedwetting laboratory findings
Cost Effectiveness of Therapy
Scientific consensus