What does "Tag" mean?

Tag:

A tag is a snippet of HTML code added to a website. Sometimes tags come from third parties and are implemented on a web page in order to make an application work on an external website. Examples include the code used to add Google Analytics, the FB Pixel, etc. onto a web page. One must be careful what happens when one places a tag on a web page, and make sure it doesn't delay the performance (speed) of page loading.

Compare with Meta Tag, a specific type of tag.

 Web Dev: Tag Best Practices