Unencrypted information that may be input to an encryption operation. Note: Plain text is not a synonym for clear text. See clear text.
Sources:
CNSSI 4009-2015 under plain text from IETF RFC 4949 Ver 2 - Adapted

  Information that is not encrypted.
Sources:
CNSSI 4009-2015 under clear text from ISO/IEC 7498-2

  Intelligible data that has meaning and can be read or acted upon without the application of decryption. Also known as cleartext.
Sources:
NIST SP 800-20 under Plaintext
NIST SP 800-67 Rev. 2 under Plaintext

  Usable data that is formatted as input to a mode.
Sources:
NIST SP 800-38A under Plaintext

  The input data to the authenticated encryption function that is both authenticated and encrypted.
Sources:
NIST SP 800-38D under Plaintext

  The input to the authenticated-encryption function.
Sources:
NIST SP 800-38F

  Intelligible data that has meaning and can be understood without the application of decryption.
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Plaintext
NIST SP 800-57 Part 2 Rev.1 under Plaintext

  Data that has not been encrypted; intelligible data that has meaning and can be understood without the application of decryption.
Sources:
NIST SP 800-175B Rev. 1 under Plaintext

  Intelligible data that has meaning and can be understood without the application of cryptography.
Sources:
NIST SP 800-175A