What is this symbol

Using Arduino Microcontrollers
#1

What is the device in the red circle to the left of the Schmitt trigger? This is from the ATMEGA328P datasheet.

Thank you,
Dave

#2

I have always assumed it is a bi-directional buffer - but I may be wrong.

...R

#3

"transmission gate." Sort of like the output stage of a DC solid state relay; current can flow through it in either direction.
Aka "analog switch." Here's an explanation: http://www.maximintegrated.com/app-notes/index.mvp/id/4243

#4

Ah so! Thank you!

#5