I want to know what's the definition of dementia?

Cognitive decline. Losses in mental functioning beyond normal aging - often short term memory may come first but other abilities may also be affected - planning, judgement, personality, temper, etc. There are several types with different profiles and they can progress fast, slowly or not at all. Alzheimer's, vascular(stroke), trauma, hiv/aids, lewey body are some very distinct types you might have heard of. Be well.
Memory problems. Memory problems, word finding problems, word substitutions errors, getting lost in places that are familiar are often early symptoms. Also forgetting how to use tools that you used to know how to use. Functional decline is part of the definition. There are many different causes of dementia and occasionally the memory problems occur later. Alzheimer's is the most common type.