Go-live is the time at which something becomes available for use. In project management or implementation, go-live is the point at which end users begin to start using a product/service. The go-live phase is also called deployment in some cases.
Typically, in implementation and onboarding projects, go-live is followed by a transition to maintenance or support teams.
The go-live is arguably the most critical milestone in any technology or SaaS project since it marks the end of a series of activities —typically spanning months—starting from requirement gathering and project planning through execution/implementation to testing and training.