Your 83-year-old uncle has always been an entertaining storyteller. In the past few years, he has been having difficulty remembering the names of people who are important for his stories. What difficulty is your uncle experiencing

Dementia.

Tip-of-the-tongue phenomenon.

Explanation:

The tip-of-the-tongue phenomenon also known as lethologica or TOT defines that the lexical access might occur in the different stages. This phenomena is quite common with increasing age.

The individual experiencing this phenomena fails to remember any word or the memory that has occurred in the life. This might also create the feeling that combined with the partial recall. The individual can recall the word immediately if they known the first word.

Thus, the answer is tip-of-the-tongue phenomenon.