clear text

    $ clear text
      1. (I) /noun/ Data in which the semantic information content
      (i.e., the meaning) is intelligible or is directly available,
      i.e., not encrypted. (See: cleartext, in the clear. Compare:
      cipher text, plain text.)

      2. (O) /noun/ "Intelligible data, the semantic content of which is
      available." [I7498-2]

      3. (D) /noun/ Synonym for "plain text".

      Deprecated Definition: IDOCs SHOULD NOT use this term as a synonym
      for "plain text", because the plain text that is input to an
      encryption operation may itself be cipher text that was output
      from a previous encryption operation. (See: superencryption.)

   $ cleartext
      1. (O) /noun/ Synonym for "clear text" [I7498-2].

      2. (I) /adjective/ Referring to clear text. Usage: Commonly used
      instead of "clear-text". (Compare: ciphertext, plaintext.)
      3. (D) /adjective/ Synonym for "plaintext".

      Deprecated Definition: IDOCs SHOULD NOT use this term as a synonym
      for "plaintext", because the plaintext data that is input to an
      encryption operation may itself be ciphertext data that was output
      from a previous encryption operation. (See: superencryption.)

plain text

    $ plain text
      1. (I) /noun/ Data that is input to an encryption process. (See:
      plaintext. Compare: cipher text, clear text.)

      2. (D) /noun/ Synonym for "clear text".

      Deprecated Definition: IDOCs SHOULD NOT use this term as a synonym
      for "clear text". Sometimes plain text that is input to an
      encryption operation is clear text, but other times plain text is
      cipher text that was output from a previous encryption operation.
      (See: superencryption.)

   $ plaintext
      (I) Data that is input to and transformed by an encryption
      process, or that is output by a decryption process.

      (C) Usually, the plaintext input to an encryption operation is
      cleartext. But in some cases, the input is ciphertext that was
      output from another encryption operation. (See: superencryption.)

2.4. Definition Type and Context

   Each entry is preceded by a character -- I, N, O, or D -- enclosed in
   parentheses, to indicate the type of definition (as is explained
   further in Section 3):
   -  "I" for a RECOMMENDED term or definition of Internet origin.
   -  "N" if RECOMMENDED but not of Internet origin.
   -  "O" for a term or definition that is NOT recommended for use in
      IDOCs but is something that authors of Internet documents should
      know about.
   -  "D" for a term or definition that is deprecated and SHOULD NOT be
      used in Internet documents.

参考

https://www.ipa.go.jp/security/rfc/RFC2828-03CJA.html#cleartext (RFC2828 の和訳)
https://www.ipa.go.jp/security/rfc/RFC2828-03CJA.html#plaintext (RFC2828 の和訳)