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In the philosophy of language, the term ordinary language is sometimes used as synonymous with natural (as opposed to mathematical or logical) language. The term has been adopted to describe computer input terms and language modeled after or based on natural human languages; rather than the artificial syntax and terms involved with computer languages. Natural language processing (NLP) is considered a sub-field of artificial intelligence and linguistics. Natural language issues hold applicability, particularly in the areas of text mining, search engines, automatic summarization, information retrieval, speech synthesis, and algorithms employed to determine the content of text streams in computational linguistics (e.g. viterbi algorithm).