A binomial expression is a mathematical expression that consists of two terms separated by a plus or minus sign. For example, x + 3 and 2y - 5 are both binomials. These expressions can represent various algebraic concepts and are often used in polynomial equations. Binomials can be added, subtracted, multiplied, or divided, and they play a crucial role in algebra. The Binomial Theorem is a key principle that helps in expanding binomials raised to a power, providing a systematic way to calculate the coefficients of the resulting polynomial.