Carryover or carry-over refers to insulin effects lasting past the insulin's official duration. It's been observed that some long-acting insulins leave an insulin depot under the skin that has a small residual effect that may last anywhere from 12 to 48 hours, after the principal action has ended. For terminology's sake, consider the principal action of the insulin to be its onset, peak, and duration. If you shoot insulin again before the duration has expired, there will be overlap. Carryover is whatever small residual effect is left after duration has expired.