Todd said: 
"Not nessecarly a coaster with no complete circut is a shuttle"
  A shuttle has to do with the lift and drop mechanism.  A boomerang is a shuttle because it is lifted and droppped into the course of the ride, look up the term shuttle in any dictionary.  It is otherwise defined as a non-continuous coaster.  
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Oooooh, my freekin' head....  Too many G's Too many G's!  No, no, Too much Beer before the G's!  Home Park:  Knott's Berry Farm