Answer: y-value of the vertex of a ∪-shaped (positive) parabola
Explanation:
For a quadratic: it is the y-value of the vertex of a positive parabola.
If it is a negative parabola ( ∩-shaped), it is the maximum.
For a cubic: it is the y-value of all of the vertices of the ∪-shaped sections of the graph. These are typically referred to as the "relative minima" when given an interval.