What is the Meaning & Definition of Turbo Lag

This "feeling of emptiness" or that "feeling of having no response on the part of the turbo" is known as Turbo Lag. But... There is neither one thing nor the other (or better said, can be explained).
The Turbo Lag is actually the delay in the response of the turbo until it reacts with effectiveness. More precisely it is that period of time that elapses between the accelerator is depressed until specifically translates the increase of pressure in feed. So, for a few seconds there are those who feel that "lack of response" and feel that the turbo does not respond or directly do not have turbo.
The explanation for this phenomenon is quite simple: when the product of the exhaust gases have to overcome the inertia of the turbine especially when you are the vehicle is detained or is circulating at very low rpm, then the operation of the engine is very little affected by the action or presence of the turbo.
Will also explain it this way: when the motor rotates a few revolutions per minute, when the final turbine rotates slowly, engine feels as if not take a turbo. This leaves happen when Gets the necessary speed so that it may compress the intake air.
In some large motors, or with very large turbocharger, move the turbine is very complicated when you are standing or walking slowly and gases needed to deal with a very strong inertia. To solve this issue using turbochargers that are increasingly smaller, built with much lighter materials, but fundamentally very resistant to heat (titanium or ceramic for example), or so-called variable geometry turbochargers.
Until not long ago in cars with turbochargers of large size, it was necessary to have the driver to step on the accelerator more account to avoid feeling that "lack of response of the turbo" or what is the same: that Turbo Lag.