Acceptance rate (or rejection rate) is the ratio of the number of articles submitted to the number of articles published. It can measure the selectivity or prestige of a journal, though like many journal metrics, the raw number is not the whole story.
There is no single list or database of acceptance rates. If the tools below don't provide a rate for a journal, emailing the journal editor or other staff is likely the best way to get that information. UlrichsWeb - Ulrich's Periodical Directory will list contact information for a journal's editorial staff.
The method of calculating acceptance rates varies. Some journals use all manuscripts received as a base for computing the rate. Others allow the editor to choose which papers are sent to reviewers and calculate the acceptance rate on those that are reviewed that is less than the total manuscripts received. Also, many editors do not maintain accurate records on this data and provide only a rough estimate.
A prestigious and multidisciplinary journal like Science or Nature will receive many more submissions than they can accommodate in publishing, regardless of merit, resulting in a low acceptance rate. A special journal on a narrow topic (like a specific disease) may only have a limited number of experts writing in the field, resulting in a higher rate. Journals that publish a limited number of paper issues would have a lower rate than one that publishes many issues online (where page counts don't significantly affect costs). Use care when comparing rates.
Many sources provide acceptance rates within particular fields or disciplines.