A triangle which has all three of its sides equal in length.

Try this Drag the orange dots on each vertex to reshape the triangle.
Notice it always remains an equilateral triangle. The sides AB, BC and AC always remain equal in length

An equilateral triangle is one in which all three sides are
congruent (same length).
Because it also has the property that all three
interior angles
are equal, it really the same thing as an equiangular triangle.
See Equiangular triangles.

An equilateral triangle is simply a specific case of a
regular polygon, in this case with 3 sides.
All the facts and properties described for regular polygons apply to an equilateral triangle.
See Regular Polygons

Properties

All three angles of an equilateral triangle are always 60°.
In the figure above, the angles ∠ABC, ∠CAB and
∠ACB are always the same.
Since the angles are the same and the internal angles of any triangle always add to 180°, each is 60°.

The area of an equilateral triangle can be calculated in the
usual way,
but in this special case of an equilateral triangle, it is also given by the formula:

With an equilateral triangle, the radius of the
incircle is exactly half the radius of the
circumcircle.

Constructing an Equilateral Triangle

It is possible to construct an equilateral triangle of a given side length using just a compass and straightedge. See
Constructing an Equilateral Triangle