Pure Data (Pd) is a free and open source visual programming language for audio and other multimedia. It is a data-flow programming language that uses "objects" connected by wires. In the tradition of modular synthesizers, Pure Data files are often known as "patches."
Identifying Pure Data patch files
Pure Data patch files use the extension .pd, a custom plaintext format consisting of newline-delimited commands. File tend to start with #N canvas and contain numerous lines starting with #X obj and #X connect.
The format is "officially undocumented" but contributors have developed a rough specification.