1.  finder(n = noun.person) - someone who comes upon something after searching;
Derived form verb find₃

2.  finder(n = noun.person) discoverer, spotter - someone who is the first to observe something;
Derived form verb find₁, verb find₂, verb find₃, verb find₆

3.  finder(n = noun.artifact) view finder, viewfinder - optical device that helps a user to find the target of interest;

1.  One who, or that which, finds; [1913 Webster]

2.  A slide ruled in squares, so as to assist in locating particular points in the field of vision. [Webster 1913 Suppl.]

finder, n.
1 a person who finds.
2 a small telescope attached to a large one to locate an object for observation.
3 the viewfinder of a camera.

Idiom:
finders keepers colloq. whoever finds a thing is entitled to keep it.