A decompiler is the name given to a computer program that performs, as far as possible, the reverse operation to that of a compiler. That is, it translates a file containing information at a relatively low level of abstraction (usually designed to be computer readable rather than human readable) into a form having a higher level of abstraction (usually designed to be human readable). The decompiler does not reconstruct the original source code, and its output is far less intelligible to a human than original source code.

The term decompiler is most commonly applied to a program which translates executable programs (the output from a compiler) into source code in a (relatively) high level language which, when compiled, will produce an executable whose behavior is the same as the original executable program. By comparison, a disassembler translates an executable program into assembly language (and an assembler could be used to assemble it back into an executable program).

Decompilation is the act of using a decompiler, although the term can also refer to the output of a decompiler. It can be used for the recovery of lost source code, and is also useful in some cases for computer security, interoperability and error correction.^{[unreliable source]} The success of decompilation depends on the amount of information present in the code being decompiled and the sophistication of the analysis performed on it. The bytecode formats used by many virtual machines (such as the Java Virtual Machine or the .NET Framework Common Language Runtime) often include extensive metadata and high-level features that make decompilation quite feasible. The presence of debug data can make it possible to reproduce the original variable and structure names and even the line numbers. Machine language without such metadata or debug data is much harder to decompile.