Citations are references to a publication from other publications in a given database. The database’s coverage thus has an impact on whether a citation counts.

Citations are increasingly being used to shed light on the use and dissemination of research (impact), and a number of indicator measures have been developed based on citations, for example.

• mean normalised
• top percentile
• field weighted citations

Standards for the calculation of these metrics can be found in the Snowball Metrics recipe book. Citations are also an element in the calculation of the h-index and a range of metrics of journal impact. 

You can only make citation analyses in databases and search engines that index the publications’ reference lists. 

The traditional citation indexes are Web of Science and Scopus, which are competing products with a considerable overlap. You can use them separately, but often it will be appropriate to carry out searches in both in order to assess which database is best in a given situation.

You should be aware that the databases primarily cover journal articles, and that there is limited coverage of humanities and social science research, for example.

Google Scholar also includes citations and has a broader coverage of publication type, topic and language. Google Scholar's interface can make it more difficult to conduct systematic searches in the same manner one can in the traditional citation indexes.