Definitions

A friend once told me that it was silly to argue over definitions.  I understand his point.  If someone starts a conversation with "Let X be blah..", there is no point arguing: "No it's not".

However, what you can argue about definitions is whether or not they are commonly accepted, or, more precisely, how commonly accepted they are. 

The other thing you can argue is the usefulness of a particular definition.  For example, defining "black" as "white" isn't particularly useful.

So, given various options for a particular definition I will favor usefulness over acceptance.