What is the data called that is to be encrypted by inputting into an encryption algorithm?

Plaintext.
The process of encoding information is called encryption. In this process, the data is converted to an unreadable/unrecognizable form so that no one can access that data except authorized parties.
Any information (a message, document, file, etc.) that is to be communicated/stored in encrypted form is called plaintext. Hence, it's a plaintext that is to be encrypted by inputting into an encryption algorithm.