What is input?

In the context of testing:

Input (input data) is the information that the tested system, application, or component receives or processes. This can include user-entered data, API request parameters, file contents, messages received from other systems, etc.

The role of input in QA is critically important because it is the variations in input data that are used to determine how the system responds and to identify defects. The diversity of input (valid, invalid, boundary, special characters) allows testing various scenarios and conditions of the system.