Glossary
Mr. Pickles the AdPlugg Pug

Impression

A basic ad impression occurs any time an ad is served.

When AdPlugg serves an ad it uses a tracking Beacon to record the serving of the ad in accordance with the international standards set forth by the IAB (The Interactive Advertising Bureau).

According to the IAB:

"A valid ad impression [is] counted when an ad counter receives and responds to an HTTP request for a tracking asset from a client"
Read the full text here: http://www.iab.net/media/file/Global_meas_guidelines.pdf