What's the meaning of algebraic data type? I'm reading a book about Haskell, a programming language, and I came across a construct defined "algebraic data type" that looks like
data WeekDay = Mon | Tue | Wed | Thu | Fri | Sat | Sun

That simply declares what are the possible values for the type WeekDay.
My question is what is the meaning of algebraic data type (for a mathematician) and how that maps to the programming language construct?

I'm sure the term comes from Algebraic Theories (as opposed to Geometric Theories), see Model Theory. The comp.sci. "ADT" is basically an object of a free model of an algebraic theory.
More details here: https://github.com/vpatryshev/wowiki/blob/master/Algebraic%20Data%20Types%20Categorically.md
