What is the defining feature of a Newtonian telescope?

The defining feature of a Newtonian telescope is that it is a type of reflecting telescope designed by Sir Isaac Newton. This telescope utilizes a primary mirror to gather light and reflect it to a focus point, where an eyepiece is positioned. In a Newtonian design, the eyepiece is located on the side of the telescope body rather than at the end. This side positioning allows for a more compact design and makes it easier to use the telescope without having to maneuver around the end of the tube.

The other options suggest designs that either do not employ a reflecting mirror system or place the eyepiece at the end, which does not align with the classic Newtonian design principles focused on usability and efficiency in light gathering and viewing angles.