According to the Oxford English Dictionary a cliche is:
1. A phrase or opinion that is overused and betrays a lack of original thought.
a. A very predictable or unoriginal thing or person
2. A stereotype or electrotype
The term ‘cliche’ comes from a French word meaning to stereotype, and that is the primary definition I want to use for this series of articles; though some of the first definition will be present.
For the purposes of fantasy literature, a cliche is a phrase, character, archetype, or plotting tool/structure that has become common in fantasy writing. Some examples of cliched characters and character archetypes are the farm boy hero, the elderly hermit mentor, the damsel in distress, the virtuous knight, or the evil overlord. Each of these has seen a great amount of use in both late myth and legend/early narrative (e.g. Tales of King Arthur), as well as in modern fantasy.
The heroic quest has also become a cliched plot device in modern fantasy because of its overuse, as has the coming of age story. Now, of course, this is far from being a complete list of fantasy cliches (there are quite a few of them after all), but it should give you the general idea of what to look for. If you are writing your first novel, and you have seen an idea repeatedly in your reading, then you should probably avoid trying to write something distinctly similar.
Of course, if we took this as absolute truth, then most of us would never write anything at all. Everything has been written before and, honestly, just about everything is cliche to someone, so we have to deal with cliches to some degree. The two things to keep in mind are 1) What is a cliche to your target audience? and 2) What can you get away with?
First of all remember that readers define cliches by what they have read before. If your audience is adolescents then you can probably get away with a lot more cliches than someone writing for an adult audience. Furthermore, if you are writing dark fantasy you have to deal with different cliches than you do writing heroic fantasy, or romantic fantasy. Your first responsibility is to know which cliches you have to deal with.
Secondly, you need to figure out what your audience will enjoy, what they will accept, what they will forgive, and what they will hate, and then you need to treat each accordingly. What you can get away with is very important, because this will be the limit of what you can actually write. Remember that.