So, ... did you read my email?
A tracking pixel, is one of various techniques used on web pages or email, to unobtrusively (usually invisibly) allow checking that a user has accessed some content. Common uses are email tracking and page tagging for web analytics. Alternative names are
1×1 gif, and
There is a work in progress to standardize an interface that web developers can use to asynchronously transfer small HTTP data from the User Agent to a web server that call it simply beacons (in the context of web development) which can be used to send data to a web server prior to the loading of the document without delaying the load and affecting the perception of page load performance for the next navigation .
The excerpt above has been retrieved from wikipedia.