Fig 1. The hand sign for Mi (pitch of E) in solfege.
Fig 2. The notes of E in different octaves on the stave, in scientific pitch notation, and on a standard piano keyboard, with frequencies shown in hertz.
Using a reference frequency of 440 Hz for the A above middle C, and calculated in equal temperament, middle E (E4) has a frequency of 329.628 Hz. The 6th (bass) string of a guitar in open tuning is tuned to E2 (82.41 Hz) and the 1st string to E4.
E also refers to the key of E major.