A vertex is where two or more line segments meet. A corner.
Plural of vertex is vertices (pronounced “ver-tiss-ease”).
A vertical X (vertex) shows the X marking the high point. But in maths it’s even more precise. It is where there are corners, and or, where two or more lines meet.
A pentagon has five vertices
A square has four corners but these are four vertices too.
Where two lines meet is a vertex.
A parabola can be thought of as two curves meeting, that is why mathematicians still call this a vertex. This can be the high point or low point of the graph.