Turbo lag is a situation when there is not enough boost buildup in the turbine. In order for the turbine to work, the engine has to produce enough exhaust gases. In most cars, the turbine kicks in at 1,500 or 2,000 RPM. So, the empty space from zero until the engine reaches this 1,500 RPM is considered to be a turbo lag. Which in other words is the waiting period until the turbo kicks in.