Definition of a lead in online advertising
A lead occurs when the campaign gives the advertiser a potential new customer or new sale. The potential customer could leave their email address or their phone number, or sign up for a newsletter, so that the advertiser could act on that information later on.
A good example of a lead is when a person registers for a test drive at a car dealer.
Both acquisitions (CPA), actions (also CPA) and orders (CPO) are often used for the same purposes, and the naming depends on the specific systems and the advertising models (for example affiliate marketing).
I also recommended using the CPM calculator to make sure you get your numbers right.