In computing, adoption means the transfer (conversion) between an old system and a target system in an organization (or more broadly, by anyone).
If a company works with an old software system, it may want to use a new system which is more efficient, has more work capacity, etc. So then a new system needs to be adopted, after which it can be used by users.
There are several adoption strategies that can be used to implement a system in an organization. The main strategies are big bang adoption, parallel adoption and phased adoption. “Big bang” is a metaphor for the cosmological theory of the same name, in which the start of the cosmos happened at one moment in time. This is also the case with the big bang adoption approach, in which the new system is supposed to be adopted wholesale on one date. In the case of parallel adoption, the old and the new system are run in parallel initially, so that all the users can get used to the new system, but still can do their work using the old system if they want to or need to do so. Phased adoption means that the adoption happens in several phases, so that after each phase the system is a little closer to being fully adopted by the organization.