from wiktionary:
Noun
obround (plural obrounds)
(geometry) A plane shape consisting of two semicircles connected by parallel lines tangent to their endpoints.
A feature or component with an obround shape.
Synonyms
stadium
discorectangle
So more than one correct answer to original (good) question, at least per this source.
PS that's pronounced "ahb-round" per above source.
"Hey babe, I'll be back in an hour, I'm going to run a few Yasso repeats on the high school obround."
Maximally filleted rectangle
It’s called a “stadium” usually.
Good answer but many indoor tracks are banked so those would be truncated oblate spheroids.
What's in a name?
In the interests of mathematical purity, we may consider points and line segments (whose ends are points) to be the basic geometric primitives from which every other shape may be constructed. Any other property of a shape may be calculated with respect to these two primitives, even continuous curvature as a limiting case.
However, since limits are typically not a good abstraction when working with geometry, we may wish to create a layer that sits on top of points and line segments which model concepts like shapes and curves, e.g., ellipses with the circle as a special case. At this higher-level abstraction, we have composable functions that allow calculations like areas and volumes of the union, intersection, and complement of common shapes.
In the specific case of the 'stadium' shape for the track, this would tend to reside at an abstraction above the one I just described, since a stadium is a simple compound shape trivially composed of the union of two circles and a rectangle. As such, I am reluctant to even give it a specific name, since it can be described very succintly in the abstraction already described.