An isosceles triangle is a triangle? in which two sides have the same length.
An equilateral triangle is an isosceles triangle because it satisfies the condition that two sides have the same length.
Although an isosceles triangle can be any way up, it is common to refer to the third side as the base.
An isosceles triangle has a line of symmetry? which passes through the midpoint of the base and the vertex? where the two sides of equal length meet.
(If the triangle is equilateral, it has other lines of symmetry as well.)
The angles at the base are equal.