Coherentism is a rival theory of justification to foundationalism. Unlike foundationalists, coherentists reject the idea that individual beliefs are justified by being inferred from other beliefs. Instead, according to coherentism, whole systems of beliefs are justified by their coherence.
What is Coherence?
Coherence consists of three elements. A belief-set is coherent to the extent that it is consistent, cohesive, and comprehensive.
A belief-set is consistent to the extent that its members do not contradict each other. Clearly a belief-set full of contradictory beliefs is not coherent. Consistency, however, need not be an all or nothing affair; beliefs may be in tension with each other, without being strictly speaking contradictory. Tensions of this kind, like contradiction, reduces the coherence of a set of beliefs.
Mere consistency is not enough for coherence. For a belief-set to be coherent, the beliefs that it contains must not only be mutually consistent, but must also be mutually supportive. A set of beliefs that support each other, where one belief makes another more probable, is more coherent than a set of unrelated, but consistent beliefs.
Finally, coherence involves comprehensiveness. Comprehensiveness, of course, is a not a part of the meaning of coherence in the ordinary sense. In the context of coherentist theories of justification, however, a belief-set increases in coherence as it increases in scope; the more a belief-set tells us about, the more coherent it is.