tolxarna'u lujvo

x1 is a real number.

mrena'u is another lujvo for the same meaning. Cf. to'e xanri namcu, mrena'u; xarna'u, lujna'u; ka'o

In notes:

frinyna'u
n1 is a rational/fractional number.
lujna'u
x1 is a complex number.
tolfrinyna'u
x1 is an irrational number.