Other forms: extendedly
Something that is extended has been fully stretched out or elongated. An extended period of time is an extremely long period of time.
To extend something is to make it longer, either physically or in terms of time. Anything extended has been stretched out in this way. An extended rubber band will snap back or break. An extended hose can be used to put out a fire. If you live with your parents in your twenties, it's like an extended childhood.