A musical concept which implies taking the long way around, harmonically. It is often used in jazz to extend a phrase or idea.

"Extended dominant" is a fancy way of referring to a dominant of the dominant relative to the key you are in. The dominant (or V7 chord) has a strong tendency to resolve to the tonic (the I chord). Hence if the composer utilizes the dominant of V7 (the 'five of five' or II7) the resolution will extend the dominant function and its musical interest -

II7  -->   V7  -->  I  (in jazz usually Imaj7) 

By doing a little math, a composer can extend the dominant function a great deal:

(In C Maj - Roots)
  
VII7 -->  III7  -->  VI7  -->  II7  -->   V7  -->  I

(B)       (E)        (A)       (D)        (G)     (C)