GNU LilyPond is a computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 for music engraving

Music engraving

Music engraving is the art of drawing music notation at high quality for the purpose of mechanical reproduction. The term music copying is almost equivalent, though music engraving implies a higher degree of skill and quality, usually for publication. Plate engraving, the process the term...

. One of LilyPond's major goals is to produce scores that are engraved with traditional layout rules, reflecting the era when scores were engraved by hand.

LilyPond is cross-platform

Cross-platform

In computing, cross-platform, or multi-platform, is an attribute conferred to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms...

, and is available for several common operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s; released under the terms of the GNU General Public License

GNU General Public License

The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

, LilyPond is free software

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

.
The LilyPond project was started in 1996 by Han-Wen Nienhuys and Jan Nieuwenhuizen, after they decided to abandon work on MPP (MusiXTeX

MusiXTeX

MusiXTeX is a suite of open source music engraving macros and fonts that allow music typesetting in TeX.-History:Macros for typesetting music in TeX first appeared in 1987 and were limited to one-staff systems....

 PreProcessor), a project they began collaborating on in 1995.
LilyPond 1.0 was released on July 31, 1998, highlighting the development of a custom music font, Feta, and the complete separation of LilyPond from MusiXTeX.
LilyPond 2.0 was released on September 24, 2003, announcing a simplified syntax model and a much more complete set of facilities for notating various styles of music.
LilyPond is mostly written in C++

C++

C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

 and uses Scheme (interpreted by GNU Guile

GNU Guile

GNU Guile is an interpreter/virtual machine for the Scheme programming language. It was first released in 1993. Guile includes modularized extensions for POSIX system calls, APL array functionality, and others packaged as an object library...

) as its extension language, allowing for user customization.