This is commonly against the rules on most servers; if it isn't here, I'd suggest it be added to the rules. With the occasional bug on the red square for traps being placed on uneven terrain, It can get pretty ridiculous.
My definition would be: "Setting traps so that any portion of the red barrier overlaps."
To Add: You can stack traps in specific ways to make a player step on as many as five traps without them being able to even see the red outline unless they do so very carefully.