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An **oval** (from Latin * ovum* 'egg') is a closed curve in a plane which resembles the outline of an egg. The term is not very specific, but in some areas (projective geometry, technical drawing, etc.) it is given a more precise definition, which may include either one or two axes of symmetry of an ellipse. In common English, the term is used in a broader sense: any shape which reminds one of an egg. The three-dimensional version of an oval is called an **ovoid**.

The term **oval** when used to describe curves in geometry is not well-defined, except in the context of projective geometry. Many distinct curves are commonly called ovals or are said to have an "oval shape". Generally, to be called an oval, a plane curve should *resemble* the outline of an egg or an ellipse. In particular, these are common traits of ovals:

- they are differentiable (smooth-looking),
^{[1]}simple (not self-intersecting), convex, closed, plane curves; - their shape does not depart much from that of an ellipse, and
- an oval would generally have an axis of symmetry, but this is not required.

Here are examples of ovals described elsewhere:

- Cassini ovals
- portions of some elliptic curves
- Moss's egg
- superellipse
- Cartesian oval
- stadium

An **ovoid** is the surface in 3-dimensional space generated by rotating an oval curve about one of its axes of symmetry.
The adjectives **ovoidal** and **ovate** mean having the characteristic of being an ovoid, and are often used as synonyms for "egg-shaped".

- In a projective plane a set Ω of points is called an
**oval**, if:

- Any line l meets Ω in at most two points, and
- For any point
*P*∈ Ω there exists exactly one tangent line t through P, i.e.,*t*∩ Ω = {*P*}.

For *finite* planes (i.e. the set of points is finite) there is a more convenient characterization:^{[2]}

- For a finite projective plane of
*order*n (i.e. any line contains*n*+ 1 points) a set Ω of points is an oval if and only if |Ω| =*n*+ 1 and no three points are collinear (on a common line).

An **ovoid** in a projective space is a set Ω of points such that:

- Any line intersects Ω in at most 2 points,
- The tangents at a point cover a hyperplane (and nothing more), and
- Ω contains no lines.

In the *finite* case only for dimension 3 there exist ovoids. A convenient characterization is:

- In a 3-dim. finite projective space of order
*n*> 2 any pointset Ω is an ovoid if and only if |Ω| and no three points are collinear.^{[3]}

The shape of an egg is approximated by the "long" half of a prolate spheroid, joined to a "short" half of a roughly spherical ellipsoid, or even a slightly oblate spheroid. These are joined at the equator and share a principal axis of rotational symmetry, as illustrated above. Although the term *egg-shaped* usually implies a lack of reflection symmetry across the equatorial plane, it may also refer to true prolate ellipsoids. It can also be used to describe the 2-dimensional figure that, if revolved around its major axis, produces the 3-dimensional surface.

In technical drawing, an **oval** is a figure that is constructed from two pairs of arcs, with two different radii (see image on the right). The arcs are joined at a point in which lines tangential to both joining arcs lie on the same line, thus making the joint smooth. Any point of an oval belongs to an arc with a constant radius (shorter or longer), but in an ellipse, the radius is continuously changing.

In common speech, "oval" means a shape rather like an egg or an ellipse, which may be two-dimensional or three-dimensional. It also often refers to a figure that resembles two semicircles joined by a rectangle, like a cricket infield, speed skating rink or an athletics track. However, this is most correctly called a stadium.

The term "ellipse" is often used interchangeably with oval, despite not being a precise synonym.^{[4]} The term "oblong" is often used incorrectly to describe an elongated oval or 'stadium' shape.^{[5]} However, in geometry, an oblong is a rectangle with unequal adjacent sides (i.e., not a square).^{[6]}