Defined by a list of comma separated domains in brackets as

tuple( 1, 2, 3)

The tuple keyword is not needed if it contains two or more domains. This can be done implicitly as shown here, or explicitly as shown here.

A domain inside a tuple can be referred to using its position index, with the first domain in position 1, and the nth domain in position n.

letting y be tuple(1,2,3)
y[1] = 1

This is demonstrated here.