An $\blue{\textbf{angle}}$ is determined by a point, two half lines in the plane that both begin at that point and one side of the plane enclosed by the two half lines. We often indicate this chosen side with a small circular arc or two short lines.

We measure the size of an angle in degrees, which we denote with $t^{\circ}$ for $t$ degrees.

An angle that goes all the way around, that is to say if the two half lines coincide and we choose the outside as the angle, is $360^\circ$. The inside of that same angle is $0^\circ$.

If the two lines are perpendicular to each other, then the angle is $90^\circ$. We call this a right angle.

When the two half lines together form one line, the angle is straight and is $180^\circ$.

An angle between $0^\circ$ and $90^\circ$ is called acute.
An angle between $90^\circ$ and $180^\circ$ is called obtuse.