How do you define the term 'call'

Hello - 

It's used everywhere in appdynamics and this might seem  like the most obvious question ever asked but I want to understand what makes up the specific defintion.    I've looked in the Glossery and throughout the documentation but I dont' see an actual defintion.   How would you define 'call'?  What makes up or constitutes a 'call'?    Without truely knowing the specific on what makes up  a call the term 'calls' or 'calls per minute' and other metrics/info dealing with CALLS leaves me a little unclear.  

Thanks