tag (noun) – a label or ticket attached to something (i.e. clothing) for purposes of identification or to offer information
tag (verb) – to touch someone with the intention of labeling them in some way (such as in the game by the same name)
NOTE: In social media, such as Facebook, Twitter, Pinterest, etc. the word tag is utilized to mean an addition, editing, or sharing of some kind of information (i.e. name, location, etc.). If you are tagged on Facebook in a photo, for example, it means that your name will appear when a person places the mouse cursor over your face in the photo (for more info on social media tagging, see the example below).
In the game of Tag one person chases others until he or she ‘tags’ or touches another person. The person who is tagged then becomes the chaser.
YouTube user Tobuscus