In *nix systems, the | character, used in pipelining.
"'The next day saw an unforgettable orgy of one-liners as everybody joined in the excitement of plumbing'"
—Douglas McIlroy
Douglas McIlroy conceived of the concept of pipelining, and it was implemented in 1973 by Ken Thompson. Pipelining is the process of sending the output of one command to the input of another. This is massively useful as it allows the user to build up quite complex structures to achieve the desired outcome. In the simple example shown below, xclip sends the clipboard contents into the wc word count utility, which the outputs the wordcount at the command line. xclip -o | wc -w | xclip -selection clipboard takes this a step further, by piping the word count back onto the system clipboard, ready for me to paste it into my writeups. This is a trivial example, I've seen some quite elaborate command lines and scripts that operate using the pipeline principle.
(An example is used in the Brevity Quest 2025 writeup: awk -F '—⍼!' '{gsub(/^by /, "", $NF); gsub(/^ \t+| \t+$/, "", $NF); print $NF}' file.txt | sort | uniq -c | sort -nr .)
$ xclip -o | wc -w
194