I'm working on an iPhone, and we are currently doing some user testing. This app deals with sensitive personal information. Security is definitely important for us. Instead of using username / password based authentication, we decided to go with 4 digits PIN/Passcode based authentication. 

User needs to input the passcode each time he invokes the app, and we don't want to hurt the usability by forcing the user to use his password each time. 

Currently, we call this passcode as PIN, because our web application uses the same terminology. User feedback suggests, our beta testers are unable to discover this feature. 

When users login for the first time, we suggest them to set a PIN, so this is not something they can miss. I believe this is because iPhone users are more accustomed to the terminology "Passcode" then "PIN". 

Maintaining consistent jargon seems like a no-brainer, but should we maintain uniformity between our products or with the system? 

If you think about it, "PIN" is a concept people should be familiar with (ATMs use the term "PIN").