Another term for the Waterfall Model methodology is the Liner Sequential Life Cycle Model methodology. It is a Software Development Life Cycle (SDLC) model that follows a sequential order in the software development process. It is a structured method and is extremely rigid. In this model, the testing phase occurs only once the development phase is completed. You cannot make changes in the requirements after the development phase begins.
Agile methodology is also an SDLC model that helps in the continuous occurrence of the development and the testing phase in the process of software development. It is flexible compared to the Waterfall methodology. The testing phase, in this model, occurs in parallel with software development. It allows you to make changes in the requirements as necessary.
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Also, watch this YouTube video on Waterfall and Agile methodology: