What is this equation called ?

[rhz_prog]

Just got some strange equation from relativistic travel problem :

$${\it PA} \left( t \right) =\lim _{{\it dt}\rightarrow 0} \left( v \left( t \right) -v \left( t-{\it dt} \right) \right) \left( 1-{\frac {v \left( t \right) v \left( t-{\it dt} \right) }{{c}^{2}}} \right) ^{-1}{{\it dt}}^{-1}$$

I use this equation to calculate the Proper Velocity of an object as a function of time PA(t) given its velocity v(t) as a function of time.

See equation RTC7683-7 in the URL below for more detail :

http://orimath.blogspot.com/2008/08/mathematical-basis-of-rtc7683.html

I want to know what this equation is called in English ?

 

[rhz_prog]

v(t) in the question above is the velocity of the object relative to an inertial reference frame as a function over time.