Two types of uncertainty.
Aleatoric (Statistical) Uncertainty
Aleatoric (aka statistical) uncertainty refers to the notion of randomness, that is, the variability in the outcome of an experiment which is due to inherently random effects.
Epistemic (aka systematic) uncertainty
Epistemic (aka systematic) refers to uncertainty caused by a lack of knowledge (about the best model). In other words, it refers to the ignorance of the agent or decision maker, and hence to the epistemic state of the agent instead of any underlying random phenomenon.
As opposed to uncertainty caused by randomness, uncertainty caused by ignorance can in principle be reduced on the basis of additional information. In other words, epistemic uncertainty refers to the reducible part of the (total) uncertainty, whereas aleatoric uncertainty refers to the irreducible part.