The term 'buffer' simply means a temporary storage place.
The buffer you're talking about is a buffer (in memory) that allows you to get at old information that had been displayed in the window. (And is currently the only use of the term within mIRC)
The buffering here is of the log that will be written to file. The fact that the buffer is a file itself is just an implementation.
Of course, if you were to refer to the buffering of the log file as opposed to the buffering of the window, you'd need two distinct terms to avoid confusion. "Log Buffering" and "Window Buffering" seem perfectly adequate, however
