An experimental language is a constructed language that explicitly explores some aspect of the effects language can have on humans. Experimental in this case refers not only to the fact that the language is created and not developed organically, but also that the language is designed to test the assumption that the way a person speaks can influence thought. In many cases, experimental languages exist only as sketches in fictional and theoretical contexts, but there are some languages of this type that are complete enough that they could be spoken. The concept most commonly associated with languages of this type is called the Sapir-Whorf hypothesis, although there are many other theories that posit some type of connection between language and thought.
When an experimental language is constructed, it is often designed with a specific goal in mind. Loglan, created by James Cooke Brown, was created to test the very idea that language and thought relate by making a language that has as little relation to existing human languages as possible. Suzette Haden Elgin also famously designed an experimental women's language called Làadan that would fit women's perceptions more fully. This feature is what sets experimental languages apart from other constructed languages. Whether the hypothesis on which the language is based is ever fully tested depends on the situation and the language's creator.
In many cases, an experimental language exists only in a theoretical or highly sketchy form. This is because this type of language is usually featured in fictional contexts where it would be inappropriate to provide a full grammar and lexicon. Given that an experimental language is necessarily a constructed language, there are also typically no native speakers for an experimental language. Problematically, this means that a person cannot truly test the idea that language affects thought, as any speaker of these languages necessarily spoke another language first.
There are many different reasons to create an experimental language, and while many of them are for fictional purposes, it is also possible to conduct useful thought experiments in linguistics using this type of exercise. When creating an experimental language, it is important to consider not only lexical elements but grammatical features as well. The way in which words divide up the world does affect the mind in many ways, but theoretically word order and other grammatical features also impact thoughts. This is a theory that is accepted to at least some degree by many linguists and other academics, but it is not universally accepted.