Mom will occasionally ask me why she has such huge lapses of memory, why is she confused about who certain people are, etc when she has moments of clarity. I don't know what to say. Do I tell her she has dementia? What have you all told your dementia people if they ask?
On a practical level, with both vascular dementia and her heart condition, I found that sketching diagrams for my mother was a good way of explaining roughly what was going on. I think my mother found this helpful partly because she always was someone with a lively curiosity about sciency things, and partly because it showed the *reason* for her ills - that there was nothing culpable, for example, in memory lapses that had a physiological cause and weren't just a failure on her part to concentrate or pay attention.
If you don't feel confident about producing a sketch there is lots of good material online. And if you're anxious about mentioning the D word, talking about brain function and physiology could be quite a useful way round that.
If mom asked about it, I might just provide whatever explanation that works in the moment. Whatever makes her feel safe. Something that she won't have to worry about.
When I first took my cousin to the doctor with her memory problem. I was pretty clueless. I knew hardly anything about dementia. So, when she started repeating things over and over, I would ask her if she remembered asking me the same question 3 minutes ago. She would say no. I knew then it was pretty serious. So, I stopped doing that. Instead, I would comfort her and say that we all have memories issues at some time or the other and that hers would improve with a better diet, medications, better diabetes control, therapy, better nutrition, etc. I kept encouraging her to enter AL, where she would get all of those things and get better. It worked too. She seemed fine with that explanation. Later, of course, she forgot all about it. She no longer knows what memory is or that she has lost almost all of hers.
Then I used the reference that the file cabinets in our brain are filled, so it takes longer to find the right file. Then that became a joke with my Dad when he couldn't remember something. "Darn file drawer is full".
I never brought up dementia because my Mom had passed from a serious fall that created late stage dementia, so I didn't want Dad to think of that. I would use the term sundowning if talking to an Aide or doctor, as Dad didn't know what that term meant, and he never asked.
My mother, on the other hand, used denial as a primary coping mechanism. We did not use the word "dementia" with her, but we occasionally had to refer to her "memory problem." If she was was worried when I was getting ready to end a visit to her nursing home that she didn't remember how to get back to her room I'd say, "It's OK if you don't always remember that. All of the helpers here know where your room is and will gladly push your wheelchair back. At your age you are entitled to let other people help you with your memory."
This is not a one-answer-suits-all question. Based on her personality, how she has coped with other problems in her life, and perhaps how severe her dementia is, do your best to figure out what approach will be less stressful for your Mom.