I think they are very similar, like regional versions of the same language. Here are a couple of quotes from Wikipedia about Bosnian: Bosnian Listeni/ˈbɒzniən/ (Serbo-Croatian: bosanski / босански; [bɔ̌sanskiː]) or Bosniak (Serbo-Croatian: bošnjački / бошњачки; [bǒʃɲaːtʃkiː]) is the standardized variety of Serbo-Croatian mainly used by Bosniaks. And: Bosnian is based on the most widespread dialect of Serbo-Croatian, Shtokavian, more specifically on Eastern Herzegovinian, which is also the basis of Croatian, Serbian, and Montenegrin. Until the dissolution of SFR Yugoslavia, they were treated as a unitary Serbo-Croatian language, and that term is still used in English to subsume the common base... Here's a link to the page I got those quotes from, in case you want to read more . https://en.m.wikipedia.org/wiki/Bosnian_language