Unit Test
In plain terms
An automated test that checks one small piece of code in isolation.
Going deeper
Unit tests verify individual functions or classes behave correctly, run fast, and pinpoint exactly what broke. They're the foundation of a healthy automated test suite.
Under the hood
Should be fast, isolated, and deterministic; over-mocking couples tests to implementation, so test behavior, not internals. Forms the test pyramid's base.
Examples
JUnit, pytest, Jest, NUnit, asserting a pure function's output.
Think of it like...
Like testing each Lego brick individually for defects before assembling the whole castle.