The protection of information and information systems from unauthorized access, use, disclosure, disruption, modification, or destruction in order to provide confidentiality, integrity, and availability.
Sources:
NIST SP 1800-10B under Information Security from FIPS 199, 44 U.S.C., Sec. 3542
NIST SP 1800-25B under Information Security from FIPS 199, 44 U.S.C., Sec. 3542
NIST SP 1800-26B under Information Security from FIPS 199, 44 U.S.C., Sec. 3542
NIST SP 1800-27B under Information Security from FIPS 200
NIST SP 1800-27C under Information Security from FIPS 200
NIST SP 800-12 Rev. 1 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-128 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-137 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-18 Rev. 1 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-30 Rev. 1 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-39 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-60 Vol. 1 Rev. 1 under Information Security from 44 U.S.C., Sec. 3542
NIST SP 800-60 Vol. 2 Rev. 1 under Information Security from 44 U.S.C., Sec. 3542
NISTIR 7621 Rev. 1 under Information Security from 44 U.S.C., Sec. 3542
NISTIR 8170 under Information Security from 44 U.S.C., Sec. 3541

  The term 'information security' means protecting information and information systems from unauthorized access, use, disclosure, disruption, modification, or destruction in order to provide integrity, confidentiality, and availability.
Sources:
NIST SP 800-59 under Information Security from 44 U.S.C., Sec. 3542 (b)(1)

  synonymous withIT Security.
Sources:
NIST SP 800-16 under Information Systems Security