Ascii85 (also called "Base85") is a form of binary-to-text encoding developed by Paul E. Rutter for the btoa utility. By using five ASCII characters to represent four bytes of binary data (making the encoded size ¹⁄₄ larger than the original, assuming eight bits per ASCII character), it is more efficient than uuencode or Base64, which use four characters to represent three bytes of data (¹⁄₃ increase, assuming eight bits per ASCII character).
Its main modern use is in Adobe's PostScript and Portable Document Format file formats.
Read more about Ascii85: Basic Idea, Btoa Version, Adobe Version, RFC 1924 Version