Alzheimer's Disease is defined by certain symptoms. However, some scientists would like to change that for research purposes, according to NPR in "Scientists Push Plan To Change How Researchers Define Alzheimer's."
Scientist's now know there are certain changes to the physical structure of the brain that can be detected in people who currently show none the symptoms traditionally associated with Alzheimer's. Changing the definition of Alzheimer's, would have no impact on how doctors currently diagnose and treat patients.
On the other hand, it would help researchers immensely. If they can conduct studies on people who do not yet have symptoms, they might be able to gain a better understanding of what causes the disease and how it progresses in the earliest stages.
Reference: NPR (April 10, 2018) "Scientists Push Plan To Change How Researchers Define Alzheimer's."