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This is the article about constructed languages. For other uses of the term "a priori", see a priori.

An a priori language is any constructed language whose vocabulary is not based on existing languages, unlike a posteriori constructed languages. Examples of a priori languages include Ro, Solresol, and Klingon Language.

Some a priori constructed languages try to categorize their vocabulary. This can be done

• Either to express an underlying philosophical system,
• To make it easier to memorize the completely new vocabulary,
• Or to remove what would be considered an unfair learning advantage for native speakers of a "source language", if the vocabulary were chosen from an already existing (natural) language.

These languages are more commonly called taxonomic languages. For example, the first letter or syllable of a word may express the class of the item (verb, noun, attribute), while the second syllable may serve to classify the word in case as referring to something alive, dead, or artificial, and so on. Thus in theory the meaning of any word can be deduced from the meanings of the individual syllables alone.

But taxonomic languages tend to be fairly awkward to use, because the classification schemes get complex quickly. For example, "apples" might be described as "red fruit". But since probably there is no category for "fruit", the term would need to be specified as something like "tree food". Moreover, "tree" itself would might appear as something like "big plant", so the word for "apple" could be the equivalent of "noun: red food from big plant" or something even more complex. In this case, the word would still be ambiguous, because cherries fit the same description.

By contrast, a posteriori constructed languages are ones whose vocabulary is based on an existing languages, either as a variation of one existing language (e.g., Latino sine flexione) or as a mixture of various languages (e.g., Interlingua).