Demonstratives are words, such as this and that, used to indicate which entities are being referred to and to distinguish those entities from others. They are typically deictic, their meaning depending on a particular frame of its reference. Demonstratives are often used in spatial deixis (using the context of the physical surroundings of the speaker and sometimes the listener), but also in intra-discourse reference - so called "discourse deixis" (including abstract concepts) or anaphora), where the meaning is dependent on something other than the relative physical location of the speaker, for example whether something is currently being said or was said earlier.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/demonstrative