According to the PMBOK Standard, requirements are part of Scope Management.
"requirements .. as clear as possible and consensus is achieved about the delivery outcome." is the activity of Requirements elicitation, which is
"Elicitation is the activity of drawing out information from stakeholders and other sources to further understand the needs of the business, in order to address a problem or opportunity and identify the stakholders' preferences and conditions for the solution that will address those needs." (Requirements Management by PMI)
To achieve clarity, a range of activities should be performed. To write 'excellent requirement', Chapter 11 of  suggest to check if requirements are
You should apply a clean writing approach, e.g. [precondition][trigger]software shall[response].
Additionally, consult IETF's RFC 2119, which contains information on requirements key words.
All this can lead you to the modelling of use cases, requirements, etc. see UML etc.
Software Requirements by Wiegers & Beatty