Before computers, plaintext meant text in the language of the communicating parties. Since computers, the definition has been expanded. It now includes not only the electronic representation of text, such as emails and word processor documents, but also the computer representation of speech, music, pictures, videos, ATM and credit card transactions, sensor data, and so forth. That is basically any information which the communicating parties might wish to hide from others. The plaintext is the normal representation of the data before any action has been taken to hide it.