A testing methodology where individual components or functions of software are tested in isolation from the rest of the system. Each test focuses on a single 'unit' of code to verify it behaves correctly.
The term 'unit' comes from Latin 'unus' meaning one, emphasizing testing single components. Unit testing practices began in the 1970s but were formalized with frameworks like SUnit (Smalltalk) in the 1990s by Kent Beck, who coined the modern approach of automated unit testing.
Unit testing is like quality-checking each LEGO brick before building a castle - you test that each brick clicks properly and is the right color before assembling the whole structure. This catches 80% of bugs at the cheapest possible moment, when fixing them costs pennies instead of thousands of dollars!
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