PACER stands for "Public Access to Court Electronic Records" and is a service that allows users to access docket and case records from US federal courts.
The website for PACER is pacer.gov, which has this description:
Public Access to Court Electronic Records (PACER) is an electronic public access service that allows users to obtain case and docket information online from federal appellate, district, and bankruptcy courts, and the PACER Case Locator. PACER is provided by the Federal Judiciary in keeping with its commitment to providing public access to court information via a centralized service.
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