It is called a "triangle"
I know, not very imaginative is it?
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Because if it were called a triangle while shaped like a square, people would laugh, and we Engineers already have enough credibility problems with clients. The same rationale applies to the musical percussion instrument.
triangle
A triangle with no equal sides is called a Scalene Triangle.
It is an equilateral triangle
this is called an obtuse triangle the triangle that has one angle 90 degrees is called a right traingle a triangle that has 3 angles less than 90 degrees is called an acute triangle