A dynamic geometry program is a program that allows the user to create, modify, and animate Euclidian geometry figures.
There are a dozen of geometry programs around, such as GeoGebra, Geometry Expressions (which also features symbolic geometry, i.e. : automatic derivation of the algebraic formulas for various measures of the geometrical objects in the model), but they all lack flexibility and thus cannot be used efficiently. For example you cannot compose simple models to obtain a more complex model and then add a button that allows the user to construct that complex model as a primitive itself.
Geometer is an ancient and interesting program but was coded in C and the DSL it uses of course is not great... The author has managed to use it for creating more than 500 illustrations for important theorems: Geometry with Computers "The book contains about 500 illustrations that can be viewed with the geometer program." Such a program would be useful for all students, teachers, scientists, engineers, programmers who code software that has to do with geometry, and also researchers who want to discover new theorems