Operator name in LL(1) computation
A set of values that comes with an "add" operation that is associative is called a semigroup.  If it also comes with an element that plays the role of 0 (so that 0 + x = x and x + 0 = x for all x), it is a monoid.

In your case, you have a monoid, as $\{\epsilon\}$ plays the role of 0 (it is an identity).