What is the ruling on a husband having sexual intercourse with his divorced wife during her 'iddah (waiting period), and is this considered as her returning to his marital bond, or is it considered zinā (fornication/adultery)?

If you divorced your wife on the condition that she relinquish her rights, then this is an irrevocable divorce (talaq bain) from which there is no return. Any intimacy that occurred between you after it is unlawful (haram), and you must separate from her and repent to Allah. If you wish to marry her again, you must do so with a new contract.

However, if the divorce was revocable (talaq raj'i) without compensation, and what was written in the document contradicts reality, then you may reconcile with her before her waiting period ('iddah) ends. In this case, your intimacy with her is considered a valid reconciliation.