Information Assurance (IA) are measures that protect and defend information and information systems by ensuring their availability, integrity, authentication, confidentiality, and non-repudiation. These measures include providing for restoration of information systems by incorporating protection, detection, and reaction capabilities.
Also see: Confidentiality, Integrity and Availability (CIA Triad)
Source: Committee on National Security Systems, CNS 4009 Glossary
Source: NYSE Assurance