OPAC stands for Online Public Access Catalog. It's essentially an online database of a library's collection that users can search to find books, journals, multimedia materials, and other resources. OPAC allows library patrons to access information about available items, check their availability, and sometimes even place holds or request materials. It's like a virtual card catalog, providing a user-friendly interface to explore and locate resources within a library's collection from the convenience of a computer or other digital device.