How is a nonlinear equation defined?

A nonlinear equation is characterized by the presence of at least one term that is not a linear function of the variables involved. This means that one or more of the variables can be raised to powers other than one, can be multiplied together, or can be involved in functions like sine, cosine, exponential, or logarithmic functions. These non-linear terms create curves or other complex shapes when graphed, deviating from the straight-line behavior that characterizes linear equations.

The other options do not accurately capture the definition of a nonlinear equation. For example, an equation where all variables are raised to the power of one describes a linear equation, not a nonlinear one. Similarly, an equation involving only one unknown variable can be either linear or nonlinear, depending on the terms involved. The notion that a nonlinear equation cannot be graphed is also incorrect; nonlinear equations can indeed be graphed, but they produce curves rather than straight lines. Thus, the option that states a nonlinear equation involves at least one non-linear term effectively encapsulates the essence of what makes an equation nonlinear.