PSR J1806 - 2125 is a pulsar discovered in the Parkes multibeam pulsar survey with a rotational period of 0.4 s and a characteristic age of 65 kyr .
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 \sim 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 .