1. Ed is a line-oriented text editor, used to create, display, and modify text files (both interactively and via shell scripts). For most purposes, ed has been replaced in normal usage by full-screen editors (Emacs and vi, for example). Ed was the original UNIX editor and may be used by some programs. However, the more modern text editors are recommended for general use. From Redhat 8.0 RPM

2. The classic unix line editor ed is a line-oriented text editor. It is used to create, display, modify and otherwise manipulate text files. red is a restricted ed: it can only edit files in the current directory and cannot execute shell commands. From Debian 3.0r0 APT

3. n. "ed is the standard text editor." Line taken from original the Unix manual page on ed, an ancient line-oriented editor that is by now used only by a few Real Programmers, and even then only for batch operations. The original line is sometimes uttered near the beginning of an emacs vs. vi holy war on Usenet, with the (vain) hope to quench the discussion before it really takes off. Often followed by a standard text describing the many virtues of ed (such as the small memory footprint on a Timex Sinclair, and the consistent (because nearly non-existent) user interface). From Jargon Dictionary