The term compile time refers to things that happen to or are discovered about a computer program at the time the compiler operates upon it; this is in contrast to run time, which denotes the time when the program is actually running on the computer.

A compiler is itself a computer program that scans, or "parses" the source code of another program and turns it into "executable code," a sequence of instructions and data the computer can execute to perform the task the program is intended perform. Depending on the language, many different things can happen at compile time.

First, in some languages like C and C++, the compiler will run what is called the "pre-processor." The C and C++ pre-processor reads in any "include files" and substitutes any macros for their associated text before allowing the compilation proper to go ahead.

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