An outmoded text editor that allowed text to be changed one line at a time. Ed was the original Unix line editor dating back to the early 1970s, and Edlin was the editor that came with DOS PCs in the early 1980s.
The first line editors were created back in the days of teletype consoles, which accepted and printed a line of text at a time. Since there were no multi-line monitors, there was no reason to be able to go up and down the lines of text in a file, and the text was manipulated by line number. In addition, dealing with a fixed line of characters required less program logic and programming skill and took less "precious" memory than expanding and compressing a variable stream of text that crossed multiple lines. See Edlin