- is a progressive neurodegenerative disorder and the most common form of dementia, characterized by gradual loss of memory, cognitive decline, behavioral changes, and eventual loss of the ability to perform daily activities. it occurs due to abnormal accumulation of protein deposits—specifically beta-amyloid plaques and tau tangles—in the brain, causing neuronal damage, inflammation, and shrinkage (atrophy) of brain tissue, particularly in areas associated with memory and cognition.