E is the third note and the fifth semitone of the C major scale, and mi in fixed-do solfège.
It has enharmonic equivalents of F♭ [(F-flat) which is by definition a diatonic semitone above E♭] and D (D-double sharp), amongst others.
[1] See pitch (music) for a discussion of historical variations in frequency.
F♭ is a common enharmonic equivalent of E, but is not regarded as the same note.
F♭ is commonly found after E♭ in the same measure in pieces where E♭ is in the key signature, in order to represent a diatonic, rather than a chromatic semitone; writing an E♭ with a following E♮ is regarded as a chromatic alteration of one scale degree.