In game programming, delta-time is:

A.
The time between when an app starts and the current frame.

B.
The required frames per second (FPS) for the app.

C.
The time when the operating system’s clock time was captured inside the app.

D.
The time in between two frames.

Explanation:

http://msdn.microsoft.com/en-us/library/bb975644(v=xnagamestudio.31).aspx