For 10 points, name this "factor" equal to one over the square root of quantity 1 minus v squared over c squared, which is used as a relativistic correction for many quantities.
The vertical half-opening angle for collimated synchrotron radiation is inversely proportional to this quantity. In CGS units, this quantity appears raised to the sixth power in a generalization of the Larmor formula named for Lienard. This quantity is equal to the derivative of the coordinate time with respect to the proper time, and it is also equal to the hyperbolic cotangent of the (*) rapidity.  Multiplying this quantity times the time interval in the stationary frame gives the time interval in the moving frame. This quantity is equal to 1 at zero velocity. For 10 points, name this "factor" equal to one over the square root of quantity 1 minus v squared over c squared, which is used as a relativistic correction for many quantities.
The vertical half-opening angle for collimated synchrotron radiation is inversely proportional to this quantity.
Lorentz gamma [or relativistic gamma or the Lorentz factor]