The expression "3x^4 + 2x^3 - x + 5" is a polynomial, which is a mathematical expression consisting of variables raised to whole number powers and coefficients. In this case, the variable is x, and the polynomial has four terms: the leading term is 3x^4, followed by 2x^3, then -x, and finally the constant term 5. Polynomials like this one can be used in various fields, including algebra and calculus, to model real-world situations. The degree of this polynomial is 4, indicating that the highest power of x is 4, which influences its shape and behavior when graphed.