Deadname
The name a trans person was given at birth and no longer uses.
A deadname is the birth name a trans person has left behind. The word is blunt on purpose: the name isn't dormant or previous, it's finished.
Using it deliberately is called deadnaming, and it lands as a refusal to accept who someone is. Doing it by accident happens, particularly for people who knew someone before. The repair is short — correct yourself, carry on, and don't make the other person manage your embarrassment.
There's a practical layer too. Old documents, bank records and employer files often keep the deadname long after the person has changed everything they can, and each one is a place they can be outed without warning.
Never ask a trans person what their deadname was. It's the single most common intrusive question, and there's no version of the answer that helps them.