Without more context, I imagine it refers to a 'two-way' switch which is commonly used, for example, to enable someone to switch a light ON at the bottom of the stairs and switch it OFF when they've climbed to the top, or at each end of a long dark corridor.

The principle is described in these webrefs:

Basic Electricity Tutorial - Switches
www.1728.org/project2.htm
This is all well and good but there are 2 drawbacks to this circuit 1) the light always stays on and 2) the .... Diagram 3 makes use of the Single Pole Double Throw Switch. ... Now someone comes to the bottom of the stairs and flips Switch 1 'up'.
What is the function of a SPDT switch? - Instructables
www.instructables.com/answers/What-is-the-function-of-a-SPD...
Oct 20, 2012 - Well, a SPDT (single pole double throw) switch is one in which there are ... the top of the stairs, flips Switch 2 'up' connecting D & E and the light ...
Multiway switching - Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Multiway_switching
Double pole, double throw switches allow control from three or more locations. ... a light from multiple locations, such as the top and bottom of a stairway, either ...
Two Way Light Switch Wiring
www.electronics-project-design.com/LightSwitchWiring.html
Have you ever wonder how a lamp that is used to light up the stairs of a ... These two way switches have a single pole double throw (SPDT) configuration.

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These wiring arrangements normally use a pair of single-pole double-throw (SPDT) switches, wired 'back-to-cak', but the system itself is usually called a 'two-way switch' (presumably because it works when moving both ways along the corridor.

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Note added at 15 hrs (2015-04-11 19:07:11 GMT)
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Ups! back-to-cak --> back-to-back, of course.