A family of Microsoft relational database management systems designed for ease of use.
In fairness, whenever I've seen this error, it's a system table that has lost its index "Id" as a result of corruption. It's not necessarily the fault of a user-designed table. All of the following system tables (normally hidden from the user) have (or should have) an index named "Id":
MSysAccessStorage
MSysAccessXML
MSysNavPaneGroupCategories
MSysNavPaneGroups
MSysNavPaneGroupToObjects
MSysObjects
MSysResources