What is Special Constable?

Legal Definition
A special constable or special police constable (SC or SPC) is a law enforcement officer.

Many police departments are complemented by a Special Constabulary which are referred to as special constables or informally as "specials". Special constables hold full police powers and hold the office of constable. Historically, and in different contexts, special constables have been paid or volunteer members of an ad hoc reserve force or a permanent auxiliary, and have ranged from unarmed patrols to armed paramilitaries.
-- Wikipedia
Legal Definition
One who has been appointed a constable for a particular occasion, as in the case of an actual tumult or a riot, or for the purpose of serving a particular process.
-- Bouviers Law Dictionary
Legal Definition
A civilian sworn in to aid a constable in preserving the peace.
-- Ballentine's Law Dictionary