  We present a fourfold series expansion representing the Askey-Wilson
polynomials. To obtain the result, a sequential use is made of several
summation and transformation formulas for the basic hypergeometric series,
including the Verma's q-extension of the Field and Wimp expansion, Andrews'
terminating q-analogue of Watson's 3F2 sum, Singh's quadratic transformation.
As an application, we present an explicit formula for the Koornwinder
polynomial of type BCn (n in Z_>0) with one row diagram. When the parameters
are specialized, we recover Lassalle's formula for Macdonald polynomials of
type Bn, Cn and Dn with one row diagram, thereby proving his conjectures.
