Worldgate
A means of fast access to other physical locations within a universe by means of a breach in the structure of spacetime. Worldgates can be naturally occurring or artificial. In the second case, the simplest way to produce them is always via wizardry.
The places where worldgates occur naturally are almost always those of unusually high population concentration. This is because the minds of living beings have an effect on the structure of the physical universe around them. The constant pressure and desire of their minds for things they want and don't have tends to "fray" universal and sub-universal structure in the area. The more beings that are packed together in one space, the more tightly they are packed, and the longer the packing is sustained, the more likely that the local structure of spacetime will fray sufficiently to tear open -- either temporarily or permanently -- and allow access to other realities. Odd disappearances of objects, or (more usually) of people, are normally the first evidence that gating is starting to happen in a given area. (This is of course something of an oversimplification of the actual physics and paraphysics, in which the population increase allows for both hyperstring "tonal frequency" shifts and the aggregation of higher-than-threshold amounts of exotic matter, sufficient to overcome the principle known locally as Hawking's chronology protection conjecture and its associated limitations on the manipulation of spacetime and "time-like curvatures".)
The population pressure required to produce a permanently resident worldgate is normally expressed by an equation known as the demobaric formula or gating pressure formula, primarily involving two interacting varables: population concentration per square meter, and size of regional population expressed as a function of planetary population. (There are other variables of much lesser importance in the equation which, for simplicity's sake, will not be discussed here.) The relationship between these variables is expressed in the threshold constant. Each planet has its own version of the formula and the constant: the relationship between them changes over time as the population of any given planet shifts.
Once the threshold constant has been reached, and the resultant worldgate has persisted for an amount of time determined by an entirely different set of variables, it will normally remain in the general area for (at least) many decades unless accidentally deranged by local physical forces, or purposely extinguished by a wizard with the necessary expertise. However, a worldgate which has persisted and remained stable for long enough will often spontaneously "spawn" and produce more gates, normally referred to as a worldgate complex, q.v.
Single or multiple, such gates seem to have a natural affinity for places where other nodes or nexi of mass transport can be found, and as a result wizards have either anchored locally occurring worldgates in in such places, or encouraged them to migrate into them. There are very few major world population centers that do not have at least one worldgate locked down in or near a major rail or air transport hub. These worldgating facilities, while effective, suffer somewhat from having to be kept under wraps in a sevarfrith culture like Earth's, and cannot be compared (except in functionality) to the great offworld facilities like the Crossings.
Wizards who need to transit from their local area to a very distant one on business -- as they often must -- will not hesitate to produce their own personal, short-duration worldgates, sometimes referred to as transit circles or by other similar names. But there's a general feeling that such energy use is irresponsible and undesirable except in emergencies. Most wizards therefore prefer to do small "short jump" personal gatings to the nearest appropriate fixed worldgate, and then use that for the longer transit, in the same way someone might walk to a bus stop, then take the bus to a train station, and the train to some more distant destination, in order to save energy and still get where he or she is going.
Both types of gating are so frequently used in the YW novels that listing them all would take quite a while. Probably most notable among personal worldgatings in the books, however, is Dairine's high-power flight across the physical universe in HW. (SYWTBAW et al.)
(See also: Worldgate complexes, Lexington Avenue Local Worldgate, Primary worldgate management team (Manhattan), Rockefeller Center worldgate, Eversion, Patency, etc etc.)