What is a Plus Icon?
A plus icon is an icon that looks like the plus sign (+) that typically symbolizes "adding" items, which in turn means "creating" new items.
It's found in buttons that "add" items to a list, to that create "new" items. On Linux, it's found in GTK's numeric text boxes instead of spin buttons, used to increment a value; and in some theme's as the maximize button.
Examples
