  We have conducted a search for giant pulses from four millisecond pulsars
using the 100m Green Bank Telescope. Coherently dedispersed time-series from
PSR J0218+4232 were found to contain giant pulses of very short intrinsic
duration whose energies follow power-law statistics. The giant pulses are in
phase with the two minima of the radio integrated pulse profile but are phase
aligned with the peaks of the X-ray profile. Historically, individual pulses
more than 10-20 times the mean pulse energy have been deemed to be ``giant
pulses''. As only 4 of the 155 pulses had energies greater than 10 times the
mean pulse-energy, we argue the emission mechanism responsible for giant pulses
should instead be defined through: (a) intrinsic timescales of microsecond or
nanosecond duration; (b) power-law energy statistics; and (c) emission
occurring in narrow phase-windows coincident with the phase windows of
non-thermal X-ray emission. Four short-duration pulses with giant-pulse
characteristics were also observed from PSR B1957+20. As the inferred magnetic
fields at the light cylinders of the millisecond pulsars that emit giant pulses
are all very high, this parameter has previously been considered to be an
indicator of giant pulse emissivity. However, the frequency of giant pulse
emission from PSR~B1957+20 is significantly lower than for other millisecond
pulsars that have similar magnetic fields at their light cylinders. This
suggests that the inferred magnetic field at the light cylinder is a poor
indicator of the rate of emission of giant pulses.
