Lifecycle is the amount of time that an electronic device is available in retail stores and has developer support. For consoles the lifecycle begins at its release and usually ends when a new version comes out. The end of a lifecycle can be a bit tricky due to the fact that even though a developer has a newer product they sometimes continue to create games and hardware for non-early adopters to enjoy. This practice is rare because most companies do not want their old systems to compete with their new one.
Lifecycle should not be confused with generation. Lifecycle is the measurement of one devices availability, while generation is the time two or more systems are on the market around the same time.