A terminal element in the WBS is any leaf element. For example, if you have the following WBS:
1- Develop website frontend
1.1 - Create Database Structure
1.1.1 - Create tables
"Create tables" would be a terminal element. As you can see, it is the same as the work package. Some project managers/references state that a work package is a different than the terminal element, but they fail to explain how. I personally see no difference between the two.