What is Sentinel?

I've heard it in Transformer movies, but it's my first time hearing it in programming.

If you search for it, it comes out as sentinel value.

https://en.m.wikipedia.org/wiki/Sentinel_value

It is a value that has a special meaning within the context of a specific algorithm. It is usually called a value that exists as a condition of ending a repeat statement or a recursion.

That's a cool term.