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In the words of Euclid:

And when the lines containing the angle are straight, the angle is called rectilineal.

(The Elements: Book $\text{I}$: Definition $9$)

Thus the distinction is made between straight-line angles and curved-line angles.

Most of the time the fact that angles are rectilineal is taken for granted.

Also known as

The term is also seen as rectilinear.