 Alternatively, it can be defined in terms of the Euler characteristic "χ", via the relationship "χ" = 2 − 2"g" for closed surfaces, where "g" is the genus.For surfaces with "b" boundary components, the equation reads "χ" = 2 − 2"g" − "b".In layman's terms, it's the number of "holes" an object has ("holes" interpreted in the sense of doughnut holes; a hollow sphere would be considered as having zero holes in this sense).