Just-In-Time Compilation (JIT) is a technique used in programming languages to improve performance by converting code into machine language at runtime. Instead of compiling the entire program before execution, JIT compiles small sections of code as they are needed, allowing for faster execution and reduced startup time. This approach is commonly used in environments like Java Virtual Machine (JVM) and Microsoft .NET Framework. By optimizing frequently executed code paths, JIT can enhance the efficiency of applications, making them run faster while still benefiting from the flexibility of interpreted languages.