A constructed language (or conlang) is any language that didn’t evolve naturally as humans moved around the world speaking to one another, but was created at a specific moment in time to serve some kind of purpose. For the purpose of this page, I’m going to divide them into three broad categories.
International auxiliary languages (also known as auxlangs) started being invented in the 19th century, as various intellectuals started thinking it would be fairer for everyone to learn a simple, neutral second language rather than one global power’s language (like French or English) be dominant with all non-native speakers having to learn it. The best-known of these is Esperanto, with approximately a million people able to speak it to some extent today, but it was neither the first nor, in my opinion, the best.
The first was probably Volapük, invented in the 1870s; it was certainly the first to “make it big”. It combined a broadly German grammar with a highly distorted English vocabulary, but declined rapidly after L.L. Zamenhof published the much more simple and regular Esperanto in 1887.
From Esperanto came a number of Esperantidos, or languages derived from Esperanto. One of these is my personal favourite, Ido (invented 1907); others include Novial (1928), Proyo (? but recent) and Mondezo (2001).
Another category of IALs is, basically, pan-Romance interlanguages, albeit often with significant English influence as well. This group includes Latino sine flexione (1903), Occidental (1922), Interlingua (1951), Romániço (1991), Lingua Franca Nova (1998), Interlingua Romanica (2001), and Neolatino (2006).
Not all interlanguages are IALs; some only aspire to be a useful interlanguage between speakers of a specific language family, or branch. Interslavic is one such language, which (as the name suggests) is designed to be a simple working language for speakers of different Slavic languages.
“Thought Experiment” Languages
I don’t know how best to title this group so I’m going to go with this. Anyway, this refers to languages which aren’t trying to be “full” languages useful for international communication, but something with a more restricted intended use.
Toki Pona is the “big name” I would put in this category. Invented by Sonja Lang in 2001, it’s a language with a vocabulary of 120–137 words (exact number varies a bit depending on the resource you check) which is intended to be minimalistic and positive. The idea is that the vocabulary only consists of the most fundamental building blocks for meaning, and by stringing those fundamental blocks together you can see what more complicated concepts are actually made up of. However, as a result, it’s a language very difficult to discuss complex topics in. There is an offshoot, Toki Ma, with a bit over double the number of words (so still not easy to discuss complicated topics in).
Láadan was designed in 1982 as a female-centric experimental language. The concept was that Western languages might be better suited to expressing the ways that men think, and that women might be better served by a different language that works how we think. There are dedicated particles to express what a sentence is (e.g. a statement, a question, a polite request, a less-polite request…) and statements are also marked for evidentiality.
These are languages invented for use in fictional stories. For example, J.R.R. Tolkien developed a family of Elvish languages that he drew on when writing Lord of the Rings. Languages like Dothraki and High Valyrian were created for the TV show Game of Thrones, building on the groundwork laid by George R.R. Martin in his novels. Star Trek is famous for Klingon.
Additionally, numerous hobbyists around the world have created languages at various times for various reasons (just having fun experimenting, or creating things to be used in role-playing games or other creative works, etc.). These are often called “artlangs”, some of which also fall into the “thought experiment” category, but others are just people having fun and not doing anything revolutionary. A long time ago, I dabbled in creating some conlangs as part of the worldbuilding for my novel, which were definitely not revolutionary but still fun to tinker with.