Ellipse

Given points $F_1$ and $F_2$, called the foci (singular: focus), and a distance $2a$ where $\text{dist}(F_1, F_2) < 2a$, an ellipse is the locus for the predicate $P(p): \text{dist}(p, F_1) + \text{dist}(p, F_2) = 2a$.