principal currency / reference currency

attitude-and-heading reference system / attitude and heading reference system (AHRS)

principal attack

principal duty

principal military requirement (PMR)

principal position

principal reserve

principal

Principal Subordinate Command (PSC)

principal direction of fire (PDF)

principal distribution depot

world map of principal aeronautical features

principal motion

principal subordinate commander

principal longitudinal axis

principal direction of fire

grid reference box

grid reference system

guidance reference system

gyroscopic reference unit (GRU)

lay reference number

map reference

map reference code

military grid reference system (MGRS)

NATO Master Cross Reference List (NMCRL) (logistics)

numbered reference position system

reference

reference diversion point

reference data

reference number (order / message / report)

stabilization reference package (SRP)

target reference point (TRP)

terms of reference (TOR)