A virtual address is an address emitted by code running when it is not running on a 'bare machine' (i.e. the machine is not using a 1:1 correspondence between the addresses used by code running in the CPU, and the actual addresses of main memory, the latter often referred to as physical addresses).
When running a time-sharing operating system, the memory management hardware provides mapping of memory addresses emitting by code belonging to a user/process, to protect both the OS itself, and other users/processes, from errors or malicious behaviour.
Originally this protection was provided by 'base and bounds registers', which allowed the entire memory of a user/process to be relocated into a contiguous block of main memory. Now, virtual memory is usually used, which provides this protection/relocation as one of its many benefits.