what are meta variables in this static analysis book's explanation about types? At page 21 of this book: https://cs.au.dk/~amoeller/spa/spa.pdf I found this:

I started reading everything and understanding it pretty well, until this.
It's defining the possible types of a language. I'm very confused about what theta and alpha mean. What are those meta-variables? What is ua.t and t[ua.t/a]?
Could someone give me some context? I'm not into computer science, this may be something specific that I have never seen before. I'm from a math background.
A metavariable is a variable that denotes some hypothetical chunk of a program. This distinction is important in the theory of programming languages, where we need to distinguish variables representing program fragments from the variables contained in those programs.
The terminology, I think, comes from logic: if you're defining a logic, you need to define it in some language that's distinct from the thing you're defining. This is the &quot;meta-language&quot;, and the variables it uses are metavariables. The history of formal logic and programming languages are deeply intertwined via the Curry-Howard correspondence, so the terminology carried over.