Is "multi-source feedback the same as 360 degree feedback?
Yes. In essence 360 degree feedback and multi-source feedback refer to the same thing.
However, there can be some slight differences in meaning. 360 feedback suggests that feedback is given that comes from various perspectives (i.e. peers, customers, suppliers, manager, subordinates), because each different source or perspective may add something unique to the feedback. That's what the 360 refers to.
Multi-source feedback doesn't have that "360 requirement", so technically, if four coworkers are giving feedback to an employee, it is multi-source feedback, but it isn't 360 feedback, because it only involves feedback from a single perspective, not a 360 perspective.