Hi Kristalyn,
Here is a summary of my understanding on this topic.
Stakeholder mapping is the larger topic where the PM analyzes various aspects of each stakeholder - their level of interest, influence, their preferred modes of communication, other specifics about them.

Stakeholder matrix is a specific type of stakeholder map, which can be combined with other stakeholder analysis techniques. A stakeholder map usually refers to the "Power-Interest Grid" (Power on the Y axis and Interest on the X axis. This is a method to visualize which stakeholder quadrants to focus on and which strategy to adopt for each quadrant: e.g.: do not expend too much energy on the low-power, low-interest quadrant, while focusing extra effort on the high-power and low-interest quadrant.

Saw this in one of my reviewer questionnaires and was searching around for what they are, the explanations lead to pretty much the same thing - the mapping of the stakeholders in the 4 quadrant chart with their level of influence and interest. It seems the map can be more than that, but it mostly points to the 4 quadrants chart. Would anyone provide some more insight?