Definition:Cosine

Analysis
$\cos x$ is voiced cosine (of) $x$, or (as written) cos $x$ (pronounced either coss or coz depending on preference).

Historical Note
The symbology $\cos$ was invented by William Oughtred in his 1657 work Trigonometrie.

Also see

 * Sine, tangent, cotangent, secant and cosecant
 * Basic Properties of Cosine Function
 * Nature of Cosine Function