Definition of an action in online advertising

An action occurs when the user gets involved with either the banner, the landing page or something else relevant to the campaign.

It depends on the campaign and its purpose what should be defined as an action.

An example of an action could be when the user clicks on a button on the landing page to open a map showing all the stores of the advertiser.

Actions as a metric is very similar to acquisitions (they both share the metrics CPA or eCPA), orders and leads.

I also recommended using the CPM calculator to make sure you get your numbers right.