1. An algebraic expression is a combination of constants, variables, and operation signs that can define a general rule or principle. It is composed of terms, where each term has a coefficient, variable, and exponent. 2. Terms are considered similar if their variable parts are the same and have the same exponent. It is important to recognize similar terms to reduce algebraic expressions by combining like terms. 3. Algebraic expressions can be entire or polynomial (having integer exponents), rational (a fraction of two entire expressions), radical (with variables under a root), or combined (containing elements of the other types).