When writing PyQGIS scripts, especially processing script and expression function, I come across the term context. I use it, but I don't know what all contexts mean actually.

There are also Qgs***Context classes. For example, QgsExpressionContext, QgsSymbolWidgetContext, QgsGeometryCheckContext, QgsAttributeEditorContext, ... .

What does context refer in all those class names and in some methods in PyQGIS?