For years, we have had terminology dividing diabetes into subgroups: there's "type 1" and "type 2" diabetes (type 1 is autoimmune diabetes with positive antibodies, typically showing up in children or teenagers, previously called IDDM or childhood-onset diabetes) (type 2 is insulin-resistant diabetes showing up in obese adults, whose antibody tests are negative, previously called NIDDM or adult-onset diabetes). There are other subgroups, but somehow these other varieties of diabetes didn't get their own numbers when the naming conventions were invented: there's "gestational diabetes" (diabetes first diagnosed during pregancy) and "secondary diabetes" (diabetes due to some other condition, such as thyroid disorders).
And there's another another type of diabetes, sometimes called LADA, that doesn't fit cleanly into either the type 1 or type 2 categories. People with LADA are diagnosed with what initially appears to be type 2 diabetes, and then rapidly progress to insulin-dependency. If diabetes antibodies (including "islet cell antibodies" and "GAD antibodies") are measured, they're positive, which is usually considered a hallmark of type 1 diabetes. LADA has sometimes been called type 1.5 diabetes (usually pronounced "type 1-and-a-half diabetes").
A few years ago (2003), an editorial in Diabetes Care discussed the name problem in an essay titled What's in a Name: Latent autoimmune diabetes of adults, type 1.5, adult-onset, and type 1 diabetes. The authors pointed out that patients with LADA have also been named type 1.5 diabetes, "slowly progressive type 1 diabetes," "latent type 1 diabetes," "youth-onset diabetes of maturity," and even LADA-type 1 and LADA-type 2. Another term that the authors didn't mention, but has sometimes been used, is "double diabetes."
In publications and in protocols for clinical trials, endocrinologists call the condition "LADA", although as jargon, both physicians and patients may call it "type 1.5". I'll use the two terms interchangeably.
Interestingly, neither the names LADA nor type 1.5 diabetes (nor any other!) have become an official part of the diagnostic lingo that physicians use to code for insurance reimbursement; the disorder probably fits best into the ICD-9 category 250.00, "Diabetes mellitus without complication type ii or unspecified type not stated as uncontrolled", rather than category 250.01, "Diabetes mellitus without complication type i not stated as uncontrolled".
What does all this mean for the average patient with diabetes? Well, first of all, if you were diagnosed with diabetes as an adult, you may assume you have type 2 diabetes, and not be aware that you have type 1.5 instead. If your diabetes was diagnosed after you became an adult, you probably didn't have antibody testing done. And if you were started on diet and exercise and your BG and A1C levels weren't controlled, you probably were started on pills, and if your BG and A1C deteriorated over time, insulin might eventually be added years after the diagnosis was first made. That's the usual scenario for people with type 2 diabetes.