The software development lifecycle (SDLC) is a structured approach that describes the stages involved in the development of software applications. It typically includes requirements gathering, system design, coding, testing, deployment, and maintenance. The SDLC is important because it provides a systematic framework for managing the software development process, ensuring that projects are completed on time, within budget, and with high quality. It helps in identifying and resolving issues early, improving collaboration among team members, and ensuring customer satisfaction.