Definitions for viewfinderˈvyuˌfaɪn dər
Random House Webster's College Dictionary
a camera attachment, as a lens and mirror that enables the operator to determine visually what will appear in the picture.
Origin of viewfinder:
finder, viewfinder, view finder(noun)
optical device that helps a user to find the target of interest
A device on a camera that shows what will appear in the field of view of the lens; it helps the user target a subject, zoom and focus the image