There is no word. The concept exists because of the word. Typically words exist to describe a concept, but in this case it's the other way around. The concept would not have otherwise existed if it were not because of the word. Therefore the concept is illusory. Made up. Created by us.

It's not worth debating sentience anymore then it is to debate at what point in a gradient does white become black.

At what point is something sentient or not sentient? "Sentience" is definitely a gradient but the point of conversion from not sentient to sentience is artificially created by language. The debate is pointless.

Here's a better example. For all numbers between 0 and 100,... at what point does a number transition from a small number to a big number? Numbers are numbers, but I use language here to create the concept of big and small. But the concepts are pointless. You may personally think everything above 50 is big, I may think everything above 90 is big. We have different opinions. But what's big and what's small is not meaningful or interesting at all. I don't care about how you or I define the words big or small, and I'm sure you don't care either. These are just arbitrary points of demarcation.

When you ask the question at what point does an AI become sentient... that question has as much meaning as asking the number question.