Acronym in science and publication: PLoS for Public Library of Science

PLoS stands for Public Library of Science. PLoS is a nonprofit organization of scientists and physicians commited to making scientific and medical literature freely available online [1]:  Every publication with PLoS is an open access publication that can be downloaded, copied, and (re)distributed.

The flagship journals of PLoS are PLoS Biology and PLoS Medicine [2-4]: PLoS Biology was the first to be launched in October 2003, followed by PLoS Medicine in October 2004.  Further domain- and platform-specific journals have been added: PLoS Computational Biology, PLoS Genetics, PLoS Pathogens, PLoS Neglected Tropical Diseases and PLoS One.

The PLoS publishing group was founded by Pat O. Brown, Mike Eisen and H. E. Varmus [4,5]. 

References
[1] Public Library of Science: www.plos.org.
[2] PLoS Biology: www.plosbiology.org/home.action.
[3] PLoS Medicine: www.plosmedicine.org/home.action.
[4] Harold Varmus: The Art and Politics of Science. W. W. Norton & Comany, New York and London, 2009; pages 146 and 161 to 165.
[5] P. O. Brown, M. B. Eisen and H. E. Varmus: Why PLoS Became a Publisher. PLoS Biology October 2003, 1(10): e36. DOI:  http://dx.doi.org/10.1371/journal.pbio.0000036.