Perimeter is a wonderful and easy topic to teach. Many teachers have slightly different definitions for the word perimeter. Most teachers would say that the perimeter is the distance around an object. I think that depending on the class level, this definition is too vague and can even be misleading.
The distance around an object or shape can be interpreted as a measure of the space just beyond the edges of the shape as seen in the picture below (which is wrong). Yikes!
I prefer to use this definition: The perimeter of a shape is the total distance along its edges. It is a definition that is much more precise students in grades 3 and over often realize this rule for themselves through hands on activities.
You may also be interested in my Unit Plan for Area and Perimeter of Squares and Rectangles available at TpT and Teachers Notebook. And I was wondering, are there any other definitions out there for perimeter?