What does "versin" mean?

Question

$$\newcommand{\versin}{\operatorname{versin}}2\versin A+\cos ^2 A= 1+\versin ^2 A$$

I don't understand the word 'ver' in this equation. What does it mean?

Answer 1

Versine is $\displaystyle1 - \cos(\theta) $ or $ \displaystyle2\sin^2\left(\frac\theta2\right)$

'versin' is the Versine here, 'ver' by itself is nothing.

It is short for 'versed sine'. See here.

Answer 2

It appears that on the unit circle, it is the remainder of the line on the radius after the cosine is subtracted for any given angle.