A "special election" is an uncommon type of election held outside of the usual election schedule. It is called to fill a vacant position or to make an important decision that cannot wait until the next regular election. Full definition
His party often sits out in special elections for assembly and state senate. (nystateofpolitics.com)
In special elections for statewide offices, the selection of the party's replacement candidate falls upon the local county committee members, who are usually citizens loyal to the Democratic establishment. (observer.com)
Some top state lawmakers seem to be changing their minds over whether to call special elections for a growing number of vacancies in the legislature. (wrvo.org)