Here are some of the abbreviations used in the text:
NCBI: National Center for Biotechnology Information
MEDLINE: MEDlars onLINE
MEDlars: Medical Literature Analysis and Retrieval System
NLM: The National Library of Medicine of the United States 
NIH: National Institutes of Health 
PubMed: Public MEDLINE – Publisher MEDLINE.
MeSH database: Medical Subject Headings database

[2] Hersh, W. Information Retrieval: A Health and Biomedical Perspective. Springer; 3rd edition (November 6, 2008) p. 124.

[3] Fact Sheet MEDLINE: http://www.nlm.nih.gov/pubs/factsheets/medline.html (accessed November 7, 2009).