What is a seal in aeronautics?
Translation

Seal means sello mecánico.

While the answer could stop here, we wouldn't know what seal types are used in turbine engines, and how some are pressurized to prevent contaminants entering the aircraft pressure vessel.



Need for bearing compartments and seals


  I've read phrases like "All gas turbine engines use pressurized seals
  for their shaft bearing compartments..."


In turbine engines, there are one or more shafts holding the rotating parts of the compressors and turbines (rotors). Bearings are inserted between a shaft and its stationary supports to limit destructive and heat-generating friction. Bearings also bear the large axial load transferred by compressor and turbine rotors to the shafts.

In turbine engine bearings, solid friction is replaced by fluid friction using an oil film between balls or rollers and races. Bearings are enclosed in a cavity and oil is injected into the cavity. This cavity is known as a sump.

The following drawing is taken from  an answer to In a turbofan what holds the spinning axis?. You can see the five bearings within their sumps:


(Source)

Oil must be circulated from the oil tank to the sumps and back, and cooled before being reused. As oil quantity is limited, oil must be prevented from leaving the oil system. This is where seals enter into action. In particular, seals are used to prevent leakage between a rotating shaft and stationary sump apertures.

High speed seals

Turbo engines' oil seals are specially selected to bear the high rotation speed of the shafts (wear), and the high operating temperature of the engine core (adjustment and bending). They are of two main types: Carbon or labyrinth. Both reduce oil pressure while it travels in the interstice between the shaft and the sump wall. 


Carbon (source) and labyrinth (source) seals

Air sealing


  All gas turbine engines use pressurized seals


As one would think, these seals are not perfect: a small quantity of oil will be able to make its way across the seal, and this quantity will increase with seal wear. Unfortunately, burnt oil releases hazardous fumes which could enter the aircraft air conditioning system as cabin air is tapped most of the time into the engine compressor.

To limit the leakage further and to recover leaked oil, a second cavity is built around the first seal and pressurized air is injected into it. This has two effects:


Air pushes oil back into the sump.
The second cavity being also sealed, any leaked oil can be drained and dumped overboard.


For example, the forward sump (on the left of the first picture) encloses 3 bearings (from left to right: #1, #2 and #3). Oil must be contained within the sump near bearings #1 and #3. Let's look at a CFM56-7B engine mounted on Boeing 737 NG. This is the lower half of the forward sump:


CFM56-7B forward sump, based on drawing from CFM56-7B Familiarization Manual


Oil enters the sump and is directed to bearing races using tubes and oil nozzles.
An oil seal prevents oil from escaping the sump in large quantity.
As some oil would nevertheless cross the seal, compressed air is injected on the other side of the seal, to counter oil leakage. Air entering the sump is then dumped overboard by the centrifugal air-oil separators.
The space where air is injected is closed by a cap on the sump with a air seal on the shaft.
Air and a small quantity of oil able to cross the oil seal are eventually drained and released overboard.
Oil crossing the bearing is scavenged and returns to the oil tank after air has been removed. 


Metal particles in scavenged oil are captured by magnets to detect abnormal wearing of bearings. From time to time oil is added to the oil tank to compensate for small leakages in the bearing sumps (and in the gearboxes).



See on Youtube: Carbon Seals - Turbine Engines: A Closer Look.