= Plane wave function
In this solution of the <Schrödinger equation>, by the <uncertainty principle>, position is completely unknown (the particle could be anywhere in space), and momentum (and therefore, <energy>) is perfectly known.
The <plane wave function> appears for example in the solution of the <Schrödinger equation for a free one dimensional particle>. This makes sense, because when solving with the <time-independent Schrödinger equation>, we do separation of variable on fixed energy levels explicitly, and the plane wave solutions are exactly fixed energy level ones.