Rujuk (husband and wife resuming the marriage during iddah)
Rujuk is when a husband takes his wife back during her iddah period, but this is only valid for a first or second talaq, which remain revocable.
A common case: a couple argue, the husband pronounces a first talaq, but during iddah they reconcile and the husband pronounces rujuk in front of witnesses.
Rujuk must be reported within a set period, and failing to do so can be an offence under the law.
Read rujuk during the iddah period, or understand the limit at talak tiga.