Requirement Engineering: ​ The broad spectrum of tasks and techniques that lead to an understanding of requirements is called ​ requirements engineering
Inception: ​ At project inception, you establish a basic understanding of the problem, the people who want a solution, the nature of the solution that is desired, and the effectiveness of preliminary communication and collaboration between the other stakeholders and the software team.
Elicitation : To help overcome problems understanding, volatility, scope, you must approach requirements gathering in an organized manner.
Elaboration ​ This task focuses on developing a refined requirements model that identifies various aspects of software function, behavior, and information..
Negotiation ​ Customers, users, and other stakeholders are asked to rank requirements and then discuss conflicts in priority. Using an iterative approach that prioritizes requirements, assesses their cost and risk, and addresses internal conflicts, requirements are eliminated, combined, and/or modified so that each party achieves some measure of satisfaction.
Specification: A specification can be a written document, a set of graphical models, a formal mathematical model, a collection of usage scenarios, a prototype, or any combination of these.

"Software validation" is the process intended to show that a system both conforms to its specifications and meets the expectations of the system customer. It involves evaluating the software to ensure that it does what it's supposed to do and that it satisfies the customer's needs and requirements.