x1 is an equal temperament dividing the interval x3 (default: octave, 2/1) into x2 (default twelve) equal parts, forming x4 (default: equal tempered semitone) as the fundamental unit.
With the places after x1 left empty, it refers to the standard twelve-equal division of the octave, but the word can also be used to refer to any tuning system which consists of fundamental units that have been obtained by dividing some interval. See also: cuvyto'aci'e.