1. A program that reads the statements written in a human-readable programming language, such as Pacal or Modula-2, and translates the statements into a machine-readable executable program. Compiled programs run significantly faster than interpreted ones because the program interacts directly with the microprocessor and doesn't need to share memory space with the interpreter. From QUECID

2. A computer program that translates high-level programs, called source files, into low-level programs, called object files. From Linux Guide @FirstLinux

3. In programming, a compiler takes human readable source code and converts it into the binary code that the computer can understand. Key point: A compiler is a form of lossy compression and one-way encryption. All the information meaningful to humans is removed from the code leaving only the information necessary for the computer. This means that humans can no longer easily read the resulting program directly. Because of the "one-way" nature of the operation, programs cannot be used to recover the existing source code. This effect is different in various languages. C++ is the worst language in terms of decompilation; Java is the best. Most Java applets can be decompiled back to some semblance of their previous form. This has led to a market for programs that further obfuscate Java binaries in an effort to hide the original source code. Some compilers do leave human-readable symbols behind for debugging purposes. They won't reveal the original source, but can still be useful for reverse engineering They can be "stripped" from the binary. From Hacking-Lexicon