Here we demonstrate that the global monsoon is the dominant mode of annual variation of the global precipitation and circulation in response to yearly solar forcing. The global monsoon can be represented by a sum of a solstitial mode and an equinoctial asymmetric mode; together, they account for 84% of the total annual variance of the precipitation and low-level circulation. The global monsoon precipitation domain can be delineated by annual range exceeding 300 mm and its ratio to local annual mean exceeding 50%. A four-parameter precipitation metrics is proposed as a metrics for evaluating reanalysis and climate models' performance. A global monsoon index and 7 regional monsoon indices are proposed to gauge variability of the global monsoon precipitation. Changes of the global monsoon in the past 56 years (observation) and in the last millennium (in ECHO-G model simulation) will be presented. The observation shows a general weakening of the global land monsoon precipitation in the past 56 years and an increasing trend over the entire global monsoon domain (including ocean) in the last three decades. The millennium simulations revealed an interesting quasi-bi-centennial and quasi-centennial oscillation as well as a fluctuation on millennium time scale in the global monsoon strength. The causes of these centennial-millennium change is discussed.