A circle or an oval (ellipse)
This depends on what one calls a "side". Usually a side needs to be straight, as in the case of polygons.
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A semicircle is one shape fitting this description.
circle
A three-sided shape with one curved side is known as a "curvilinear triangle." This shape consists of two straight sides and one curved side, creating a triangle-like form with a rounded edge. It can be used in various mathematical and design contexts, where a blend of linear and curved geometry is required.
a sphere has no bases and it only has one curved surface
An arc (segment of a circle) has one straight side, and one curved side.