Cost Per Action or CPA (sometimes known as Pay Per Action or PPA; also Cost Per Conversion) is an online advertising pricing model, where the advertiser pays for each specified action – for example, an impression, click, form submit (e.g., contact request, newsletter sign up, registration etc.), double opt-in or sale. Direct response advertisers consider CPA the optimal way to buy online advertising, as an advertiser only pays for the ad when the desired action has occurred. The desired action to be performed is determined by the advertiser. Radio and TV stations also sometimes offer unsold inventory on a cost per action basis, but this form of advertising is most often referred to as “per inquiry”. Although less common, print media will also sometimes be sold on a CPA basis.