When we say that something changes sign, we mean that it switches from being positive to negative or vice versa. In calculus, this refers to when the value of an expression or function goes from having one sign to another.
Related terms
Zero Crossing: The specific point where the sign changes from positive to negative or vice versa.
Turning Point: A point on the graph of a function where there's an abrupt change in direction.
Inflection Point: A point on the graph of a curve where concavity changes sign.