Algebraic measure of the orthogonal projection on one of the sides of an angle of measure θ of a unit vector carried by the other side of the angle.

\(\cos{(\textit{θ})}\space = \space \frac{x}{1}\space = \space x\)

When we associate with any real number the cosine of the angle whose measure, in the circular system, is this number, we thereby define a circular function called a

**cosine function.**### Notation

The cosine of the angle *x* is noted as cos(*x*) and is read as “cos of *x*” or “cosine of angle *x*“.

### Example

Graphic representation of the function *f*(*x*)= cos(*x*) with amplitude 1 and period 2π.