Medical science owes much to the development of the clinical research method known as the clinical trial (CT), which plays a key role in modern clinical research and is considered to provide the best level of evidence in evidence-based medicine. Carefully conducted clinical trials are fastest and safest way to find treatment that work in people and way to improve health. Clinical trials aim to measure therapeutic effectiveness and constitute an important and highly specialized form of biological assay. This paper is intended for people who need to understand Clinical research & clinical trial terminology and is directed in particular towards Health care professionals, nurses and pharmacists reading clinical trial reports. The coverage of material is meant to assist with understanding what clinical trials involve – it covers the majority of terms that are likely to be encountered and is a phases of clinical trials. A variety of medical, ethical terms & definitions are included. It is hoped that this paper will assist readers who understand little or nothing of terms relating to clinical trials to appreciate their essential meanings. References are provided for readers to follow-up the complete details of all the terms used.