The only definition about requirements that most people agree with is the definition of a Functional Requirement: it is a capabilty that  a system must provide, as per my previous example.

What comes before FRs is commonly called Business Requirements, but there are many different definitions, styles and formats for those.

I use Business Processes broken down to Activities and Steps to describe the business in enough detail to be able to determine a Functional Requirement.

A step may be "Employee receives and records the Customer's Order".  Then I ask: do you want system to do any of this? If the answer is yes, the step is the source of the Functional Requirement --> FR:  The system must be able to record a Customer Order.