Tell me about a person believing the neighbor constantly enters her home and steals (and it's totally not true). Is this a type of dementia?

Have her checked for a UTI. They can cause havoc and DO NOT have the same symptoms for seniors. We know because of physical discomfort, they don't get that.

If she has dementia, this could be further decline and you probably should be looking at not letting her live alone. It has to be terrifying to think, true or not, that someone is coming in your home. She must spend a lot of time looking in closets and under beds. Poor thing.

If this is your Mom, you mention she has a form of Dementia.This is a symptom. Paranoia goes along with it. If this person is living alone, they shouldn't be.

Yes.

my Dad believed that someone was stealing his mail...coming into the house and moving so he could not find it.

he also believed my Mom (age 90) had a boyfriend coming over every night after he went to bed.

Mom told him...”I already have one man too many in my life”!

No point in arguing about it. Laugh. Schedule an appointment to be sure this isn’t a sign of something much worse or something that can be fixed.

It is a delusion, which is definitely associated with some forms of dementia. If this person is living on their own they probably shouldn't be, the delusions will not be the only thing going wrong.
Hm, I just read your profile, if this is your mother then you might want to rule out a UTI, which causes some people to go off the rails.