Tuple is the formal mathematical term for what is written in formulae as (a,b,…,z) — a finite ordered collection of (possibly very different) elements. The Tcl counterpart is lists.
When all elements are of the same "type", it is more common to speak of it as a vector than a tuple.
The length of a tuple is the number of elements in it, and "n-tuple" is short for "tuple of length n". There are 2-tuples (pairs), 3-tuples (triplets), 4-tuples (quadruplets), 5-tuples (pentuplets), etc., and of course also 1-tuples and 0-tuples, although mathematicians rarely bother about the latter unless they are being very formal.
See also: