a regular oval shape, traced by a point moving in a plane so that the sum of its distances from two other points (the foci) is constant, or resulting when a cone is cut by an oblique plane that does not intersect the base.
ellipsity is the name of a NYC tech startup, combining the grammatical ellipsis (meaning the omission of superfluous words) with the common english suffix -ity (the quality of) because they want to bring back into the world what traditional advertising has omitted and overlooked. Things formerly believed to be superfluous to advertising like caring about consumers, smaller cos, society, non-profits are not superfluous and are instead the very foci of ellipsity. It may also be a misspelling of ellipticity.
You can think of it as an oblong circle. If you look at a circle from a side (i.e., not straight from above), the shape it looks like is an ellipse.