"Lag" can be applied to any length putt (whereas "lag putt" usually implies a lengthy first putt) and is often used as a verb, or after the fact to describe the second, shorter putt that results after not making the first putt. Example of verb usage: "I need to lag this putt up close" or "Just try to lag this one up by the hole." Example of an after-the-fact usage: "Nice lag," or "way to lag it up close."
Lag putting is something that can be practiced by focusing on distance control (a k a speed control) in your putting. For examples, see:
Putting drills for distance control
Chi Chi's "Lag-nificent" lag putting drill