What is the difference between a megalith and a trilithon?

A megalith is defined as a very large usually rough stone used in prehistoric cultures as a monument or building block.

Source: http://www.merriam-webster.com/dictionary/megalith

A trilithon is defined as an ancient stone monument using two upright megaliths carrying a third as a lintel.

Source: http://www.merriam-webster.com/dictionary/trilithon

In other words, it appears that a megalith is a single stone and a trilithon is a structure formed from megaliths.