Google Scholar is a freely accessible search engine that indexes the full text and metadata of scholarship across an array of publishing formats and disciplines. Released in November 2004, the Google Scholar index includes most peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, abstracts, technical reports, and other scholarly literature, including court opinions and patents.
Google Scholar's data is not perfectly accurate and is known to exclude some citations of works from some sources and include false positives that count as citations. The system's information is constantly being updated and changed, which explains why citation counts can fluctuate over time (and even go down!). But, even though the data is not perfect, Google Scholar still offers a view into a scholar's overall publication record over time and provides a rough count of total citations and growth over time viewed by calendar years.
Google Scholar "Cited By" and "Citations Per Year" as of February 2023.