Mole fraction is a way to express the concentration of a component in a mixture. It is defined as the ratio of the number of moles of a specific component to the total number of moles of all components in the mixture. This value is always between 0 and 1, where 0 indicates that the component is absent, and 1 means it is the only component present. For example, in a mixture of gases, if there are 2 moles of gas A and 3 moles of gas B, the mole fraction of gas A would be 2/(2+3) = 0.4. Mole fraction is useful in various fields, including chemistry and engineering, for calculations involving gas laws and solutions.