Intercooler: A radiator plumbed into the air flow between the compressor and the intake manifold that cools the air coming from the compressor. A cooler charge of air is denser, so more of it can be packed into the combustion chamber. Intercooler design typically falls into one of two types-air-to-air or water-to-air-depending on what means is used to do the cooling. Air-to-air intercoolers are simpler, smaller, and lighter, though they may not cool as effectively as water-to-air units.
Pop-Off Valve: (See Blow-Off Valve)
Trim: A measure of the size of the compressor and/or turbine wheel.
Turbine: The finned wheel on the exhaust side of the turbocharger that is driven by the flow of exhaust gas coming from the engine and through the turbine housing. The turbine wheel drives the shaft that, in turn, spins the compressor wheel.
Turbo Lag: A hesitation in the production of boost. Lag is typically experienced with turbochargers with large housings or large turbine wheel sizes, as it takes more exhaust pressure to get the turbine spinning (spooled up).
Wastegate: A valve used to bleed off excess exhaust pressure on the turbine side of the turbocharger. This is done to maintain optimal boost pressure without over-spinning the turbine wheel. Wastegates are either internal (built into the turbine housing) or external (plumbed downstream of the turbo). Internal wastegates are found on most OE turbochargers, as they're less expensive to make and are easier to package underhood. Internal wastegates, however, are typically smaller than external versions, and so cannot relieve as much pressure as the external units can. Some wastegates can also be adjusted to tune boost levels.