Stands for "Cost Per Lead," and is used in online advertising. CPL defines how much revenue a publisher receives when he creates a lead for an advertiser. For example, the publisher may place an ad for an investment site on his website. If a user clicks on the advertisement link, she is directed to the advertiser's website where she can sign up for an investment account. If she chooses to sign up, a lead has been created and the publisher is paid a certain amount based on the CPL.
CPL and CPA (cost per action) are often used interchangeably, though CPL is more specific.