The middle eight is that part of a song which is not a
verse, a
chorus, an
introduction or a
bridge (although the two are virtually identical, a
bridge is often played to fill a gap between each verse and chorus, whilst a middle eight stands on its own); it typically comes towards or just after the middle of a song, and may or may not last eight bars (or a multiple thereof). It is often occupied by an instrumental solo and can simply consist of a repeat of an instrumental repeat of the verse, but the classic middle eight is an entirely new piece of music. Middle eights are typical of classic rock/pop songwriters;
drum'n'bass artists do not go over large on such things.
For an example, the section of The Beatles' 'I Want to Hold Your Hand' which goes "And when I touch you I feel happy inside / It's such a feeling that my love / I can't hide, I can't hide, I can't hide" (which Bob Dylan supposedly heard as 'I get high') is a middle eight, and a good one too; the 'Pour your misery down on me' section of Garbage's 'Only Happy When it Rains' is an example more familiar to modern youth. Middle eights have provided some of the best moments in music ever.