Cost per acquisition (CPA), also known as pay per acquisition (PPA) and cost per conversion, is an online advertising pricing model where the advertiser pays for each specified acquisition - 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 acquisition has occurred. The desired acquisition to be performed is determined by the advertiser. Radio and TV stations also sometimes offer unsold inventory on a cost per acquisition 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.