Let P be a point, not the center, in the interior of a (round) disk D⊂ℝ² and let A and B be points on ∂D such that the line segments AP and BP have equal length. Choose an arc AB. What's the shape bounded by the arc and the two segments called?
I suppose you could call it an isosceles curvilinear triangle, although that doesn't capture the curvy part being an arc of a circle.
It's called a circular segment.
Edited to add: As Jasper has pointed out in the comments, this is not what the question asked. So I don't know the answer.