Interlingua (not to be confused with Peano’s Interlingua, aka. Latino sine flexione, or Interlingue aka. Occidental) is a constructed language largely designed by Alexander Gode and first published by the IALA in 1951. It is a naturalistic conlang designed to be ultra-comprehensible to people who already speak one of the Romance languages. Upon its release, it siphoned away a lot of the people who had previously been using Occidental (which lost momentum on the death of its creator). It seems to me like Interlingua and Ido are together vying for the position of third-most spoken conlang in the world (after Esperanto and Interslavic).
While Interlingua’s proponents make a fuss out of how easily comprehensible Interlingua is to non-speakers, it is reportedly a lot harder to learn than more regular conlangs, including even Occidental/Interlingue which is also naturalistic. This is partly because of Occidental’s use of de Wahl’s rule, which is a rule for regularly deriving nouns/adjectives/etc. from verbal roots; Interlingua doesn’t make any attempt at regular derivation rules, instead just borrowing words straight-up. Interlingua also has a very large vocabulary for potential learners to study; any word that appears in three of Interlingua’s four source languages (French, Italian, Spanish/Portuguese – which are counted as one language – and English; if a word appears in two of the four plus German or Russian then that is also acceptable) is permitted as an Interlingua word. Like in other conlangs, it doesn’t have to be the root word itself that appears in the source language; it can be in an adjective or something, but it must appear somewhere with that meaning. If semantic drift means a cognate now has a different meaning (like how English actually does not mean the same as Romance actualmente), that does not count as the same word for this purpose.
I have seen some people argue that while Interlingua is a bit crap as a proposal for an international auxiliary language, it would actually be fantastic as a pan-Romance interlanguage (similar to what Interslavic is for the Slavic languages). Because the Interlingua community at large doesn’t accept the premise that their language is not good as an IAL, though, it includes some influences from other languages (mostly English) that make it not-so-ideal as a Romance interlanguage either (like the lack of grammatical gender, verb conjugation, or the subjunctive). Based on this argument that Interlingua’s more of a Romance + English interlanguage, I’ve seen some others argue that it’d make a good interlanguage for the Western Hemisphere.
Like seemingly every other conlang ever, there have been proposals to make an “improved” Interlingua. I’ve seen at least one person mention Stanley Mulaik as someone who has proposed improvements to Interlingua…but haven’t really found anything that concisely sums up his proposal.