To define a user story in agile, start by focusing on the end user and their needs. Keep the user story simple and concise, usually following the format of "As a [user role], I want [goal] so that [reason]." This format helps to clearly identify who the user is, what they want to achieve, and why it is important to them. Next, prioritize user stories based on the value they provide to the customer or business. This helps in determining which user stories should be worked on first and which can be deferred.

Collaboration is key in defining user stories in agile. Involve the development team, product owner, and stakeholders in the process to ensure a shared understanding of the requirements. Use techniques like user story mapping or story slicing to break down larger user stories into smaller, manageable pieces. This allows for incremental delivery and feedback, which is a core principle of agile development.

Lastly, make sure that user stories are estimated appropriately using techniques like story points or t-shirt sizing. This helps in planning and prioritizing work during sprint planning sessions. Regularly review and refine user stories as more information becomes available or priorities change. By following these steps, teams can effectively define user stories in agile and deliver value to customers in a timely manner.