From AMS Glossary
Same as analysis, except for two important practical differences.
First, it is not done in real time, and second, the background field is made by an NWP model that does not change over the entire period of the reanalysis. A serious problem with climate change studies made from standard analyses results from frequent changes in the model used to generate the background field. These changes (including changes in resolution and orography) lead to discontinuities in a time series of real-time analyses. A reanalysis yields complete, global gridded data that are as temporally homogeneous as possible. Reanalysis data include many derived fields (heating, soil moisture over land, etc.) for which direct observations are nearly absent.