Xquartz
From wikinotes
X11 server for MacOS.
Install
Configuration
# Do not start xterm on xquartz start defaults write org.macosforge.xquartz.X11 \ app_to_run /usr/bin/true
X11 server for MacOS.
# Do not start xterm on xquartz start defaults write org.macosforge.xquartz.X11 \ app_to_run /usr/bin/true