Encompasses the configuration, usage, and control of additional displays / monitors / screens. Often termed dual-monitor or dual-head (for two monitors), and often in involving external displays. Connection methods include VGA, DVI, HDMI, and DisplayPort. For single displays, use "monitors" instead. For directing a program's graphical output, you may also want the "display" tag.
The multi-monitor tag encompasses the configuration, usage, and control of two or more screens. Scenarios include a primary display (often built-in, as with laptops) and one or more additional displays. The method of attachment varies, and should be mentioned in your question.
For Linux-based systems, useful information also includes your video card details (
lspci | grep VGA), monitor details (
lshw -c display), and current driver versions (
grep driver /var/log/Xorg.0.log). Other useful commands include xrandr and