One's subcutaneous "spare tank" of insulin, which has yet to be used by the body. Because no insulin injection is immediately 100% absorbed by the body, the yet to be used insulin stays under the skin, the system drawing on this "reserve" as needed. Any such insulin effects that last after the insulin's expected action is over are also known as carryover in some literature. If you're using only intermediate-acting or long-acting insulin alone (without any short-acting insulin), about 24 hours worth of insulin requirements are in your depot. That's a brilliant awsenr to an interesting question