A Great Comet is a comet that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great Comets are rare; on average only one will appear in a decade. While comets are officially named after their discoverers, Great Comets are sometimes also referred to by the year in which they appeared great, using the formulation "The Great Comet of...", followed by the year.
The vast majority of comets are never bright enough to be seen by the naked eye, and generally pass through the inner solar system unseen by anyone except astronomers. However, occasionally a comet may brighten to naked eye visibility, and even more rarely it may become as bright or brighter than the brightest stars. The requirements for this to occur are: a large and active nucleus, a close approach to the Sun, and a close approach to the Earth. A comet fulfilling all three of these criteria will certainly be spectacular. Sometimes, a comet failing on one criterion will still be extremely impressive.
For a comet to become spectacular, it also needs to pass close to the Earth. Comet Halley, for example, is usually very bright when it passes through the inner solar system every seventy-six years, but during its 1986 apparition, its closest approach to Earth was almost the most distant possible. The comet became visible to the naked eye, but was definitely unspectacular. On the other hand, the intrinsically small and faint Comet Hyakutake (C/1996 B2) appeared very bright and spectacular due to its very close approach to Earth at its nearest during the March of 1996. Its passage near the Earth was one of the closest cometary approaches on record.