Zero-day

A cyber capability that relies on a vulnerability in the design or implementation of a system and can be used to violate its security. Neither the system designers, cyber security community, or general public are aware of the vulnerability.

Source: Cyber Threats to Elections – A Lexicon, Cyber Threat Intelligence Integration Center & Office of the Director of National Intelligence, https://www.dni.gov/files/CTIIC/documents/CTIIC_2018_Lexicon_without_banner_small_file_for_Post.pdf