MeSH is a great resource,a thesaurus, especially significant if you access Medline — from whatever source.  (PubMed is the free version, accessible everywhere anyone has access to the internet.)

MeSH means Medical Subject Headings.  They are assigned by indexers at the National Library of Medicine.  This thesaurus is complete with definitions or scope notes.  It is organized in a hierarchical fashion so that if you wanted to search all antibiotics, for example, you wouldn’t have to separately type all of them, but could just “explode” the main heading.  You can limit the headings to a major concept, the most important concept(s) in the article.  You can also attach subheadings, such as adverse effects of antibiotics.