With Campaign Attribution, there are two ways to define a Response. If you are configuring Salesforce, a Response is a campaign member value that you set up. If you are reviewing data, a Response is a Lead or Contact interaction. View details below for more clarification.
Response as a Salesforce Value
From a technical perspective a Response is a campaign member in Salesforce with a member status that is configured with the HasResponded flag as true. This can be configured in Salesforce by using the Advanced Settings on any Campaign.
Response as an Interaction
A Response indicates an interaction with a person (Lead or Contact). For example, an email that was sent but never opened would not be considered a Response. You determine what counts as a Response, within your business process. Some companies will consider an open or a click a Response, while others will require additional action such as a form fill before considering the interaction a Response.