Wiktionary
n. (context chemistry English) the ratio of the number of moles of one component of a mixture to the total number of moles
Wikipedia
In chemistry, the mole fraction or molar fraction (x) is defined as the amount of a constituent (expressed in moles), n, divided by the total amount of all constituents in a mixture, n:
$$x_i = \frac{n_i}{n_\mathrm{tot}}$$
The sum of all the mole fractions is equal to 1:
∑n = n; ∑x = 1
The same concept expressed with a denominator of 100 is the mole percent or molar percentage or molar proportion (mol%).
The mole fraction is also called the amount fraction. It is identical to the number fraction, which is defined as the number of molecules of a constituent N divided by the total number of all molecules N. The mole fraction is sometimes denoted by the lowercase Greek letter χ ( chi) instead of a Roman x. For mixtures of gases, IUPAC recommends the letter y.
The National Institute of Standards and Technology of the United States prefers the term amount-of-substance fraction over mole fraction because it does not contain the name of the unit mole.
Whereas mole fraction is a ratio of moles to moles, molar concentration is a quotient of moles to volume.
The mole fraction is one way of expressing the composition of a mixture with a dimensionless quantity; mass fraction (percentage by weight, wt%) and volume fraction ( percentage by volume, vol%) are others.