In summary, MAP = (Systemic vascular resistance) × (Cardiac output). We are interested in this variable because it is the pressure which seems to have the greatest influence on bloodflow autoregulation within organs, and on whole-body haemodynamic homeostatic mechanisms (such as the baroreceptors). It is resistant to confounding measurement factors, and is attractively easy to calculate. However, it has a series of drawbacks, not the least of which being the lack of agreement regarding what an appropriate MAP is in any given situation. The golden "MAP of 65" rule is another one of those things which has widespread support and absolutely no evidence.
MAP = (Systemic vascular resistance) × (cardiac output).
MAP = geometrical mean of the invasive arterial blood pressure waveform; or the mathematical adjusted mean of the non-invasive measurement
NIBP MAP = (diastolic pressure) + (pulse pressure divided by 3)
Relevance to hemodynamics
Resistance to confounding factors
Disadvantages of MAP as a haemodynamic goal
Axler, Olivier. "Measuring Mean or Systolic Arterial Pressure in Critically Ill Patients? Invasively or Noninvasively? Does It Matter?*." Critical care medicine41.1 (2013): 333-334.
Li-wei, H. Lehman, et al. "Methods of blood pressure measurement in the ICU."Critical care medicine 41.1 (2013): 34.