Note: mat catastrophe points out quite correctly that NASA material now refers to this as a Transoceanic Abort Landing.
In NASA and Space Shuttle terminology, this is the second intact abort mode available to the Shuttle in the event of a failure during ascent. It is considered marginally preferable to a Return to Launch Site abort, because the shuttle isn't required to reverse course during ascent and boost - a risky maneuver.
Just as it is named, this abort procedure involves the Shuttle continuing across the Atlantic Ocean and, after jettisoning the Solid Rocket Boosters and External Tank, gliding to a landing at a preplanned site in Africa or Spain. Early in the Shuttle program, the preferred landing sites were U.S. military bases in Spain at Zaragosza and Moron. Since then, NASA has added potential landing sites to the list. Which is to be used would depend on launch trajectories, the time of the failure, and the condition of the landing field at launch time. The complete list now includes:
At some point, Easter Island was also in the running, although I don't know if this option was ever actually planned out.
This option is available from the moment of the last possible RTLS opportunity through a point called Single-Engine Press to MECO - a point in the ascent where the orbiter (according to NASA) "...has sufficient velocity to achieve main engine cutoff (MECO) and Abort To Orbit, even if two main engines are shut down."
This abort mode is available to the crew after that point if conditions dictate it (structural damage preventing exoatmospheric operations, for example). Although a manual abort mode early in the Shuttle Program, it has since been added to the menu of automatic abort options. I don't know if the Shuttle is capable of piloting itself to a landing during such an abort, but I'd tend to doubt it; the Shuttle computers would probably be hard-pressed to reconfigure themselves quickly enough. With the new glass-cockpit upgrades, however (which flew for the first time aboard Atlantis in early 2001), this might be possible. The Soviet Buran shuttle flew itself to a wheel-stop landing on its maiden (and only) flight, after all.
TAL does not require any OMS burns by the Shuttle to carry out. This might be a deciding factor in choosing between this option and Abort Once Around or Abort To Orbit procedures.