Definitions will be found ...

Definitions will be found where a term originated. However, some simple descriptions may help understanding a term. I shall attempt to state such descriptions. 

Globalisation   is conceiving or implementing a process that shall extend across all political boundaries. "Think globally, act locally" is a theme popularised by UNEP.

Decentralisation   is development of systems that combat a problem near its source.  For example, in the arrangement for treating sewage, decentralisation will mean a sewage treatment plant for a housing estate.

Cross-level approach  is an attempt to deal with the issues related to a problem strategically to overcome the complexity associated with the problem.