The relationship between from fields and to fields when performing append, merge or similar operations indicating how fields will be populated in the result dataset.
Questions should use this tag if they are concerned with populating fields of different names during an amalgamation.
Operations that amalgamate data with mismatched fields (not the same name) need to have field mapping applied so that the geoprocessing operation can populate the correct fields from various source datasets.
Normally ArcMap tools like Append and Merge will attempt to field map using the first dataset as a template; when executing these operations from python (arcpy) the field map must be presented textually prior to version 10 or as a FieldMap object or only the default (name/type) match will be performed.
Field mapping in an Esri sense is discussed in the Esri online help.