Just-in-time compilation*

A technique used in interpreted or bytecode-compiled languages where program code is compiled at runtime, during evaluation. This gives the speed advantages of compilation, while retaining the dynamic flexibility and architecture-independence of a interpreted or bytecode-based language. Can sometimes give even better performance than static compilation, since more information may be available at runtime, leading to better optimisation. Heavily used in modern implementations of JVM and .NET.

[Wikipedia]