American Sign Language: "monitor"

MONITOR (observe):

MONITOR (computer monitor):

Draw a square in the air with your index fingertips.

Note: If you do this sign "larger" it can mean "board" as in white board or blackboard.

"SQUARE-perimeter" can be used to indicate a billboard or display sign.  This is a very "general" sign used to indicate whatever is "square shaped."

Add black to it and you have: BLACK + BOARD 
Add White to it and you have:  WHITE + BOARD

square / board / sign

Blackboard