  We have measured the frequency of the $6s^2S_{1/2} - 5d^2D_{3/2}$
electric-quadrupole transition of $^{171}$Yb$^+$ with a relative uncertainty of
$1\times 10^{-14}$, $\nu_{Yb}$ = 688 358 979 309 312 Hz $\pm$ 6 Hz. A
femtosecond frequency comb generator was used to phase-coherently link the
optical frequency derived from a single trapped ion to a cesium fountain
controlled hydrogen maser. This measurement is one of the most accurate
measurements of optical frequencies ever reported, and it represents a
contribution to the development of optical clocks based on an $^{171}$Yb$^+$
ion standard.
