The medical term lethargy refers to a state where it is difficult to arouse. 

The child doesn’t care what is going on around them, or for that matter, what is  being done to them.    

This is a worrisome sign and warrants prompt evaluation.

Many parents use the term more loosely, referring to decreased activity as lethargic. At times there have been children touted to be lethargic who are running around the office; when asked why they think their child is lethargic the parents state that they are no longer running and jumping at the same time. 

When we refer to lethargy, we are speaking of pronounced lack of activity, a child who is difficult to awaken.