While there are many different definitions for what makes a classic novel, it is most commonly agreed that classics are novels of literary significance that have withstood the test of time and remained popular years after their publication. Generally, they contain wide, global themes that can be applied to any time period. A classic usually contains some kind of widespread, universal appeal that results in it being read and embraced by a wide audience of diverse people. Usually, it also contains some unique artistic quality, be it a brilliant storyline or an engaging writing style, that sets it apart from other works of literature.
There is much debate regarding what makes a classic novel; there is by no means a well-defined set of rules to define a work as a classic. Traditionally, a classic is a work written in ancient Greek or Latin, but in modern terms, however, the term may be applied to any work that is a strong model of its form. Most widely-observed classic novels are at least several decades old and still widely read. Some people do apply the term to very recent novels and call them "modern classics."
There is also debate regarding what styles of novels can be considered classics, and traditionally, only standard fictional narratives could be included. In recent years, however, some graphic novels have been referred to as classic novels. Some people react to this with shock, as graphic novels are seldom taken seriously by literary authorities. Some graphic novels do, however, meet the common criteria: widely-read fictional narratives that have powerful universal themes and have remained popular for decades.
While a classic usually has some unique quality that sets it apart from other works of its time, it also tends to build off from the literary trends of the past. Writers tend to be widely-read and quite knowledgeable, so it is not uncommon to find influences from other writers and other significant artistic works in their novels. The history of literature and of the themes addressed by a classical novel usually play a significant part in the work as a whole.
Regardless of the definition used, it is a great honor for a novel to be referred to as a classic. Classic novels tend to touch people and to help them define the world around them. Their confrontation of major universal themes makes people think and encourages them to form their own opinions of those themes. Perhaps it is this level of influence more than anything that truly defines a classic work.