The "plain text" term has a different meaning.
In cryptography, plaintext is information used as input to an encryption algorithm; the output is termed ciphertext. The plaintext could be, for example, a diplomatic message, a bank transaction, an e-mail, a diary and so forth — any information that someone might want to prevent others from reading. Typically the plaintext is the message in a form where no special effort has been made to make the information unreadable without specail knowlage. In some systems, however, multiple layers of encryption are used, in which case the ciphertext output of one encryption algorithm becomes the plaintext input to the next.
Secure handling of plaintext
In a cryptosystem, weaknesses can be introduced through insecure handling of the plaintext, allowing an attacker to bypass the cryptography altogether. Plaintext is vulnerable in use and in storage, whether in electronic or paper format. The area of physical security deals with how media can be secured from local attacks. An attacker might enter a poorly secured building and attempt to open locked desk drawers or safes. An attacker can also engage in dumpster diving, and may be able to reconstruct shredded sheets. One countermeasure is to burn or thoroughly crosscut shred discarded printed plaintexts. (See Paper shredder for specifications.) If plaintext is kept in a computer file, the disk along with the entire computer and its components must be secure. Sensitive data is sometimes processed on computers whose mass storage is removable, in which case physical security of the removed disk is vital. In the case of securing a computer, that security must be physical (e.g., against burglary, brazen removal under cover of a repair, installation of covert monitoring devices, etc.) as well as virtual (e.g., operating system modification, illicit network access, Trojan programs, ...). The wide availability of keydrives, which can plug into most modern computers and receive hundreds of megabytes of data, poses another security headache. A spy (perhaps posing as a cleaning person) could easily conceal one and even swallow it, if necessary.
Discarded computers, disk drives and media are also a potential source of plaintexts. Most operating systems do not actually erase anything — they simply mark the disk space occupied by a deleted file as 'available for use', and remove its entry from the file system directory. The information in a file deleted in this way remains fully present until overwritten at some later time when the operating system reuses the disk space. With even low-end computers being sold with 30 Gigabytes of disk space, this 'later time' may be months, or never. Even overwriting that part of a disk occupied by a deleted file is insufficient in many cases. Peter Gutmann of the University of Auckland wrote a celebrated paper some years ago