A brief glossary and description of some terms is provided below. Further detail can be found from a paper by the Casualty Actuarial Society here
ELT : a table with event details from the RMS catastrophe model.
YLT : a table with event losses by year and event
OEP : probability of a loss of a given size or larger in a year
AEP : probability of total losses in a year of a given size or larger.
Primary Uncertainty : whether an event happens or not
Secondary Uncertainty : given an event happens the uncertainty in the size of the loss