Optical Viewfinder
A finder that shows the scene by direct optical path rather than on a screen.
In depth
An optical finder has no lag, no refresh rate and no power draw, and it presents the scene at the full dynamic range and colour fidelity of the eye. What it cannot do is show the effect of the chosen exposure, white balance or picture profile, so judging those remains an act of prediction confirmed on the rear screen.
Examples in practice
Tracking a swallow in flight through an optical finder shows the bird in real time, with no electronic delay between eye and subject.