Is every Isosceles Triangle an equilateral triangle?

NO but every Equilateral triangle is an Isosceles triangle. That is the difference .

Isosceles triangles have only 2 sides the same length.

Equilateral have ALL 3 sides the same length which means that they must have 2 sides the same length, so they are a very special case of an Isosceles triangle.

That is why they have a special name - Equilateral meaning 'all sides equal'. The Word lateral is a reference to the Word length.