A pulsar with a pulse period of less than 25 milliseconds, equivalent to an axial rotation rate of 40 revolutions per second. Such a high spin rate may suggest that these pulsars are young, but in fact the opposite is true.
Millisecond pulsars are typically a billion years or more old.
Indeed, millisecond pulsars are so regular that their pulses can be averaged to create the most accurate clock known to man. Of more than 700 pulsars so far identified, fewer than 20 are of the millisecond variety.