A virtual assistant (or VA) is a software program which is (1) not required within a specific context, but which (2) can help a specific agent to perform one or more activities within that context.
Virtual assistants can be distinguished from other software programs which are required by a specific role.
For example, an agent may need to use a specific software program to communicate with other agents in a specific digital network. However, that agent may use a virtual assistant to help them communicate more effectively and efficiently. (The virtual assistant may or may not interact directly with other, required programs.)
Some virtual assistants may be able to function as bots. VAs which are bots may act as delegates for agents within some contexts.