Having a better understanding of Late might lead to the discovery of new treatments, say the researchers.
They have written guidelines to help increase awareness and advance research into the newly defined disease.
Lead author Dr Pete Nelson, from the University of Kentucky, said: "It's been there all along obviously but we are just first recognising it.
"Alzheimer's disease is something that everyone knows about - it's a way of getting dementia - but there are different diseases as well and this disease we are calling Late is one of the most common, and so it is sort of an exciting time to begin and do research and trying to optimise and develop therapies.
"There is a lot of work to be done. This is more of a starting point than a finish point.
"There's no doubt that many people who were previously called Alzheimer's, in fact, did not have Alzheimer's."
What do other experts think?
Prof Robert Howard from University College London said: "This is probably the most important paper to be published in the field of dementia in the last five years.
"Treatment trials of drugs that are designed to work against Alzheimer's will not have any efficacy against Late and this has important implications for the choice of participants in future trials."
Prof Tara Spires-Jones, an expert in dementia at the University of Edinburgh, said: "This paper is important because we know that dementia symptoms can be caused by many underlying diseases, and it is essential to understand what causes the diseases in order to develop targeted treatments."
Dr James Pickett, from the Alzheimer's Society, said the work was "the first step towards more precise diagnosis and personalised treatment for dementia".
Dr Carol Routledge, from Alzheimer's Research, cautioned that it wasn't something that doctors would be able to diagnose in the clinic yet.
Researchers are working to find diagnostic tests and targeted treatments for different types of dementia.