The collectoin of RunTime information for later use at CompileTime.

The profiling information is generally saved to disk, either as a summary (requires more RAM) or as a log (requires more disk) and then used to identify parts of the code which would most benefit from Optimisations of various types. HotSpotCompilers do this all AutoMagically at runtime.

GCC uses the -pg argument to build excutables with profiling enabled and gprof(1) to display the resulting data. Java can profile normally compiled classes, because the profiling infrastructure is built into the JVM, there is a standard interface to the profiling and debugging features and several user-level interfaces, including fancy GUI ones.