Medical terminology is composed of a prefix, root word, and suffix:
Prefix: A prefix is placed at the beginning of a word to modify or change its meaning. Pre means "before." Prefixes may also indicate a location, number, or time.
Root: central part of a word.
Suffix: The ending part of a word that modifies the meaning of the word.

• FamilyDoctor.org Dictionary

• Medical Dictionary of Health Terms - Harvard Health Publishing

• MedlinePlus Medical Encyclopedia

• RX List Dictionary

• WebMd Online Medical Dictionary

• Biopharmaceutical Glossaries - Cambridge Healthtech Institute

• Dictionary of Cancer Terms - National Cancer Institute (NCI)

• National Comprehensive Cancer Network (NCCN) Dictionary

• NCI Metathesaurus

  A wide-ranging biomedical terminology database that covers most terminologies used by National Cancer Institute for clinical care, translational and basic research, and public information and administrative activities. Search box at the top right.

• Introduction to Medical Terminology - U of Minnesota

• Word Parts and What They Mean - MedlinePlus

• Understanding Medical Words Tutorial - National Library of Medicine

• Word Building Reference - Medical Terminology and word building

• CDC Diseases and Conditions

• Mayo Clinic Diseases and Conditions

• MedicineNet Diseases and Conditions

• Medline Plus Medical Encyclopedia

• MeSH (Medical Subject Headings) - National Library of Medicine

• Lab Tests Online

  Health information web resource designed to help patients and caregivers understand the many lab tests that are a vital part of medical care.