A visitor to the Baldwin Museum of Science website signs up to receive its newsletter. What is this customer action called?
In paid advertising marketing, a conversion is any valuable action the use performs. Depending on your goals and business type it could be a purchase, a download, a sign up, a call or similar. In order to measure conversions and first you should configure conversion tracking.
In this case, if Baldwin Museum of Science has set up conversions tracking and their goal is to generate leads (newsletter sign-ups) such customer action will be counted as a conversion.
To measure the actual impact of your PPC campaigns, you should always set conversion tracking. Also, add a conversion value to better measure and optimize your campaigns.