What do you call this symbol?

Question

I saw this in a textbook used to describe a parameter set, but haven't been able to find a name for it.

Anyone know it?

PS. Darker background simply the textbook color, not part of the symbol.

Answer 1

It's an upper case greek letter theta: $\Theta$ (in LaTeX/MathJax \Theta).

The lower case variant (in LaTeX/MathJax \theta): $\theta$, also sometimes written as $\vartheta$ (in LaTeX/MathJax \vartheta), which more resembles what the letter looks like in hand writing.

Answer 2

Looks to me like an uppercase Greek letter theta ($\Theta$)