  PSR J1806-2125 is a pulsar discovered in the Parkes multibeam pulsar survey
with a rotational period of 0.4s and a characteristic age of 65kyr. Between
MJDs 51462 and 51894 this pulsar underwent an increase in rotational frequency
of $\Delta \nu/\nu \approx 16 \times 10^{-6}$. The magnitude of this glitch is
approximately 2.5 times greater than any previously observed in any pulsar and
16 times greater than the mean glitch size. This letter gives the parameters of
the glitch and compares its properties to previously observed events. The
existence of such large and rare glitches offers new hope for attempts to
observe thermal X-ray emission from the internal heat released following a
glitch, and suggests that pulsars which previously have not been observed to
glitch may do so on long timescales.
