Apex angle (vertex angle) of an isosceles triangle

An apex angle is the angle between the lines that define the apex. 

Apex angle of an isosceles triangle.

In an isosceles triangle, the apex is the vertex where the two sides of equal length meet, opposite the unequal third side

 The angles at the base are called the base angles, and the angle opposite the base is called the vertex angle.

An isosceles triangle  is a triangle that has two sides of equal length.

The two equal sides are called the legs and the third side is called the base of the triangle.