I remember asking the question in grad school, "What is the difference between a language delay and a language disorder?" My professor did not give us a clear answer. In the end I was led to believe that they were the same. Then I kept encountering more labels! It gets very confusing. What is acceptable? What is used? What are the differences? What do they mean? I'm hopeful that this post will help to clear up some of that confusion.
Language Delay -This term is used by many SLPs but should probably be avoided because it tends to imply that the child is simply 'a little behind' and will catch up soon without any intervention. It's probably a good idea not to use it unless you have really examined the child and feel strongly that it truly describes the child. This term would only seem logical to use for a child who is 2 or 3 years old. An older child who is still struggling with language is less likely to 'outgrow' it.
Language Deviance -This term doesn't seem to be used very often, but it implies that the child's language development is different somehow. If you use this term you need to know what is different and if that difference is a disorder or not. Which brings us to the next term:
Language Difference - This term is often used and is acceptable as long as it is used correctly. This means that the child has obvious differences in their use of language but they do not have a disorder. It is usually used to describe children who are learning English as a second language.
Language Disability/Language Disorder/Language Impairment - These three are used interchangeably and are considered to be the most acceptable terms used when describing a child who is definitely having difficulty learning language. Their language scores on tests are below the acceptable range and others notice that they have difficulty following directions or expressing themselves.
Specific Language Impairment (SLI) - First introduced by Kamhi (1998), specific language impairment is used to describe a child who has no other difficulties such as lower IQs or syndromes or birth defects. It is considered a 'pure' language impairment and these children tend to be quite rare. It is very difficult to separate language and cognition. Which is a totally different post. SLI is the term I saw used most frequently in the school system, and it was used regardless of whether or not the child had other challenges going on.
Developmental Language Disorder - Is the best term to use when describing a child who has low language abilities as well as other problems such as low IQ's or syndromes such as Down's Syndrome or Autism. When I worked in the schools, I didn't see it used very often but I like the idea of having a name for a language disorder that is coexistent with other cognitive problems.