JUnit
In plain terms
The standard unit-testing framework for Java.
Going deeper
JUnit provides annotations and assertions to write and run automated Java tests, foundational to Java testing culture. It popularized xUnit-style testing.
Under the hood
JUnit 5 added a modular architecture and richer features; integrates with build tools and CI.
Examples
Java unit tests, @Test annotations, assertions, Maven/Gradle, TDD.
Think of it like...
Like the standard ruler and gauges every Java workshop uses to check its parts.