The image seen in a plane mirror can't be formed on a screen. What name is given to an image of this type?

Any image that can be formed on a screen are called real image. Such images are formed by light rays emanating from the object, when reflected and/or refracted by devices such as lenses and mirrors converge at some specific point.

In contrast thee are other images which are formed by the rays of light reaching the observer appearing to converge at some point. This happens when the rays reaching the observer are diverging and when the line of these rays are projected backward these lines converge. Such images images, which  cannot be formed on a screen, are called virtual images.