Generally, a data citation refers to an entry for a dataset within the reference list of an article, book, conference proceeding, or other document. Datasets can also be cited when they are in a repository. Datasets are often not listed within the references, however, and may sometimes be noted in the methods section or the acknowledgements. While this can be considered a data citation, they will not be captured by standard citation counting methods, and so will not be included in databases that track citation counts.
Data citation from figshare:
Schiavo, Jennifer (2020): Innate and plastic mechanisms in auditory cortex for maternal behavior. figshare. Collection. https://doi.org/10.6084/m9.figshare.c.5043830.v6
Data citation from ICPSR: Kovner, Christine Tassone, and Brewer, Carol S. Newly Licensed Registered Nurse Quality Improvement Survey 1, 2008. Inter-university Consortium for Political and Social Research [distributor], 2021-01-27. https://doi.org/10.3886/ICPSR36817.v3
The metrics toolkit provides information about definitions, use cases, and limitations for this and many other metrics: https://www.metrics-toolkit.org/metrics/citations_data/
Force11 is an organization that supports scholarly communications and they worked with other organizations to develop a Joint Declaration of Data Citation Principles: https://force11.org/info/joint-declaration-of-data-citation-principles-final/