= M =

Major topics (Medline / MeSH)

Asterisks on MeSH headings and subheadings (e.g., Wound Healing/radiation effects*) designate that they are the major topics of the article, usually obtained from the title and/or statement of purpose

Non-major (non-asterisked) headings and subheadings are usually additional topics substantively discussed within the article, terms added to qualify a major topic (…).

The only indexed MEDLINE citations without an asterisked heading are some biographies in which the subject’s name may be considered the only major point. (…)

Source : http://www.nlm.nih.gov/bsd/disted/mesh/major.html

Mapping

In resources with controlled vocabulary, you can map search terms to subject headings

Source: OVID help. Available form http://www.ovid.com/site/help/documentation/ospb/en/advanced.htm#map