MeSH = Medical Subject Headings. MeSH is an example of a Controlled Vocabulary.
MeSH terms are added to the citations in MEDLINE (Ovid or PubMed) to enhance retrieval. This is, in addition to searching the terms that authors' used in their titles and abstracts, you can also search on the MeSH terms assigned (or tagged) by the indexers at the National Library of Medicine.
MeSH terms are grouped by topic or subject, e.g. Anatomy; Heart Diseases; Occupational Groups. The benefit of this is that you can "explode" on a broad MeSH term to broaden your search by include all of the more specific terms. Great way to get a class of things without having to key them all in.
MeSH terms can be narrowed by using subheadings. The subheading terms let you specify which facet of an idea that you want. e.g. economic; manpower; history; statistics and numerical data.
MeSH terms can also be narrowed down by selecting Focus (Ovid) or Major MeSH (PubMed). Focus means that the MeSH you select is the main idea of the article, not simply a minor idea.
It can take from 1-3 months for a citation in MEDLINE to be tagged with a MeSH term. We recommend that you use both MeSH and keywords if you are looking for the most current information.
For more info on MeSH check here:
Links to MeSH database, terms & descriptions: http://www.nlm.nih.gov/mesh/meshhome.html
PDF handout on how to use MeSH in searching: http://nnlm.gov/training/resources/meshtri.pdf