Systematic reviews are studies that collectively evaluate primary studies, which are usually original research articles, published on a particular topic. Systematic reviews and meta-analyses are considered secondary sources of information. In this respect, it would not be right to include the information obtained from previous systematic reviews and meta-analyses in systematic reviews.

An abbreviation is a shortened version of a word or phrase. For example aa (amino acid), d (day), hr (hour), mg (milligram)

Acronyms are abbreviations formed by combining the first letters of words and read as a single word. For example AIDS (Acquired Immune Deficiency Syndrome), LASER (Light Amplification by Stimulated Emission of Radiation).

An initialism is the words formed by combining the first letters of the words, but in which the abbreviated letters are read separately. For example DNA (deoxyribonucleic acid), pH (potential of hydrogen-original Latin pondus hyrogenii), RNA (ribonucleic acid). In some grammar books, initialism is included under the title of abbreviations.