x1 is a pure imaginary number (such that its real part is 0) understood as an example of a complex number.
Includes 0 = (0,0) = 0 + 0i. This word relies on xo'ei implicitly filling the second slot of "ka'o" and also relies on the Cartesian decomposition of all complex numbers into real and imaginary bases (due to ka'o). See also: ka'o zei namcu and ka'ozeino zei namcu.