Enharmonic

The musical term enharmonic describes a note or notes which are the same, but whose spelling is different, or changed upon subsequent appearances. For example, a pitch written as an E♭ in one measure might be written as a D♯ in the next. On a piano, these note names represent the same pitch, so composers have a reason for chaning the spelling.

Generally speaking, a sharp or flat, especially when it's not part of the piece's key signature, implies a sense of direction as to where it is leading. In this sense, an E♭ wants to resolve downwards to a D, but a D♯ wants to resolve upwards to an E. So when a composer changes the spelling of a note, they are indicating to the performer that the note's function has changed. Almost invariably this follows a drastic shift in harmony, making this practice one technique in which composers can change from one key to another, also known as enharmonic modulation.