A pulsar that flashes every few thousandths of a second, i.e. with a period of 1–10 ms. The first to be discovered, PSR 1937+21, has a period of 1.56 ms. Even faster is a pulsar called PSR J1748-2446ad, in the globular cluster Terzan 5, which has a period of 1.40 ms, very close to the theoretical minimum for a spinning neutron star. Around 300 millisecond pulsars have been discovered, many of them in globular clusters. Millisecond pulsars are extremely stable rotators and keep time better than atomic clocks. See also Recycled Pulsar.