Capable of being observed; discernible; noticeable.

describing a characteristic or behavior that is visible to its stakeholder. For example information hiding makes software less observable, and therefore less testable because the tester needs to be able to observe the actual characteristic or behavior in order to compare it with the expected characteristic or behavior.

capable of being seen or noticed; "a discernible change in attitude"; "a clearly evident erasure in the manuscript"; "an observable change in behavior"

Noteworthy; remarkable.

a self-adjoint operator

Worthy of being observed; important enough to be noted or celebrated; as, an observable anniversary.

Some physical property of a system that can be observed.

In physics, particularly in quantum physics, a system observable is a property of the system state that can be determined by some sequence of physical operations. These operations might involve submitting the system to various electromagnetic fields and eventually reading a value off some gauge. In systems governed by classical mechanics, any experimentally observable value can be shown to be given by a real-valued function on the set of all possible system states.

an object that holds some data