Binomial theorem is a mathematical formula that expands the power (x + y)^n of a binomial expression (x + y) into a sum of terms. Each term in the expansion is a combination of x and y raised to a power, multiplied by a coefficient that is determined by the binomial coefficients (n choose k). The theorem is expressed as: (x + y)^n = C(n, 0) x^n y^0 + C(n, 1) x^(n-1) y^1 + C(n, 2) x^(n-2) y^2 + … + C(n, n-1) x^1 y^(n-1) + C(n, n) x^0 y^n