Shared Database
In plain terms
Multiple services accessing one common database.
Going deeper
The shared-database pattern has several services read and write the same database, which is simple but tightly couples them. Changes to the schema ripple across services.
Under the hood
An anti-pattern for microservices (couples services via schema); database-per-service is preferred for autonomy.
Examples
Legacy systems, tightly-coupled services, monolith data, migration challenges.
Think of it like...
Like several teams editing one shared spreadsheet, convenient until they step on each other.