Simultaneous operations which may or may not share data, events, and resources. Typically used to describe computing systems in which multiple tasks are operating off of the same processing core; the tasks are assigned time slices, priority, or operation types to get them to all play together.
This term is very often misused in business and society by people who are trying to "do two things at once" without understanding the basic nature of a multi-tasking environment. Thusly, we get the kind of discussion below:
Start refactoring at MultiTask.
See: OperatingSystems