Due to copyright restrictions, any books defining terms, such as dictionaries and glossaries, effectively restrict further publication of those definitions, unless permission is acquired from the original publisher. To avoid the economic consequences, producers of new dictionaries and glossaries must ensure that the definitions that they use are somewhat different from those of other publishers. A definition is intellectual property. Variant definitions are therefore constantly produced to avoid infringing on the property rights of others, thus destabilizing the knowledge base.